Tag Archives: storage

Choosing a Storage Type

While technicalities defining which type of storage is which can become problematic, the underlying concepts are pretty well understood.  There are four key types of storage that we use in everyday server computing: local disks, DAS, NAS and SAN.  Choosing which we want to use, in most cases, can be broken down into a relatively easy formula.

The quick rule of thumb for storage should be: Local before DAS, DAS before NAS, NAS before SAN.  Or as I like to write it:

Local Disks -> DAS -> NAS -> SAN

To use this rule you simply start with your storage requirements in hand and begin on the left hand side.  If local disks meet your requirements, then almost certainly they are your best choice.  If they don’t meet your requirements move to the right and check if DAS will meet your requirements.  If so, great, if not continue the process.

That’s the rule of thumb, so if that is all you need, there you go.  But we will dive into the “why” of the rule below. The quick overview is that on the left we get speed and reliability at the lowest cost.  As we move to the right complexity increases as does price typically.  The last two, while very different, are actually the most alike in many ways due to their networked nature.

Local Disks:  Local drives inside your server chassis are your best bet for most tasks.  Being inside the chassis means the least amount of money spent on extra containers to hold and power the drives, least physical risk, most solid connection technologies, shortest distance and least amount of potential bottlenecks. Being raw disks, local disks are block devices.

Direct Attached Storage:  DAS is, more or less, local drives housed outside of the server chassis.  The server itself will see them exactly like any other local drives making them very easy to use.  DAS is simple but still has extra external containers and extra cables.  This adds cost and some complexity.  DAS makes it easier to attach multiple servers to the same set of drives as this is almost impossible, and always cumbersome, with local disks.  So DAS is effectively our first type of physically sharable storage.  Being identical to local disks, DAS is a form of block device.

Network Attached Storage: NAS is unique in that it is the only non-block device from which we have to choose.  A NAS, or a traditional file server – they are truly one and the same, is the first of our technologies designed to run over a network.  This adds a lot of complication.  NAS shares storage out at the filesystem level.  A NAS is an intelligent device that allows users over the network to easily and safely share storage because the NAS has the necessary logic on board to handle multiple users at one time.  NAS is very easy for anyone to use and is even commonly used by people at home.

Storage Area Network: SAN is an adaptation of DAS with the addition of a network infrastructure allowing the SAN to behave as a remote hard drive (block device) that an operating system sees as no different from any other hard drive attached to it.  SANs require advanced networking knowledge, are surrounded by a large amount of myth and rumor, are poorly understood by the average IT professional, are generally complex to use and understand and because they lack the logic of a NAS they effectively expose a hard drive directly to the network making it trivially easy to corrupt and destroy data.  It is, in fact, so easy to lose data on a SAN due to misconfiguration that the most commonly expected use of a SAN is a use case for which a SAN cannot be used.

Of course there is much grey area.  What is normally considered a DAS can be turned into a SAN.  A SAN can be direct connected.  NAS can be direct connected.  Local storage can act as either NAS or SAN depending on configuration such as with a VSA (Virtual Storage Appliance.)  Many devices are simultaneously NAS and SAN and the determination is by configuration, not by the physical device itself.  But in generally accepted use, the terms are mostly straightforward.

The point being that as we move from left to right in our list we move from simple and easy to difficult and complex.  SAN itself is a rock solid technology; it is the introduction of humans and their tendency to do dangerous things easily with SAN that makes it a dangerous storage technique for the average user.  As with everything in IT, keeping our technologies and processes simple brings stability and security and, often, cost savings as well.

There are many times when movement to “the right” is necessary.  Local disks do not scale well and can become too expensive to maintain for certain types of larger deployments.  DAS, likewise, doesn’t scale well in many cases.  NAS scales well but being a non-block protocol is a bit unique and doesn’t always work for our purposes, a good example being HyperV that requires a block device for storage.  SAN is the final catchall of storage.  If nothing else works, SAN is always there to fall back on – or, as I like to say, SAN is the storage of last resort.

This is a very high level look at the basics of choosing a storage approach.  This is a common IT task that must be done with great regularity.  I did not intend this post, in any way, to explain any deep knowledge of storage but simply to provide a handy guide to understanding where to start looking at storage options.  Exceptions and special cases abound, but it is extremely common to simply skip the best option and go straight to considering something big, expensive and complex and rapidly forget that something much more simple might do the same job in a far superior manner.  The underlying concept is the simplest solution that meets the need is usually the best.

Hot Spare or a Hot Mess

A common approach to adding a layer of safety to RAID is to have spare drive(s) available so that replacement time for a failed drive is minimized.  The most extreme form of this is referred to as having a “hot spare” – a spare drive actually sitting in the array but unused until the array detects a drive failure at which time the system automatically disables the failed drive and enables the hot spare, the same as if a human had just popped the one drive out of the array and popped in the other allowing a resilver operation (a rebuilding of the array) to begin as soon as possible.  This can bring the time to swap in a new drive from hours or days to seconds and, in theory, can provide an extreme increase in safety.

First, I’d like to address what I personally feel is a mistake in the naming conventions. What we refer to as a hot spare should, I believe, actually be called a warm spare because it is sitting there ready to go but does not contain the necessary data to be used immediately.  A spare drive stored outside of the chassis, one that requires a human to step in and swap the drives manually, would be a cold spare.  To truly be a hot spare a drive should be full of data and, therefore, would be a participatory member of the RAID array in some capacity.  Red Hat has a good article on how this terminology applies to disaster recovery sites for reference.  This differentiation is important because what we call a hot spare does not already contain data and does not immediately step in to replace the failed drive but instead steps in to immediately begin the process of restoring the lost drive – a critical differentiation.

In order to keep concepts clear, from here on out I will refer to what vendors call hot spares as “warm spares.”  This will make sense in short order.

There are two main concerns with warm spares.  The first is the ineffectual nature of the warm spare in most use cases and the second is the “automated array destruction” risk.

Most people approach the warm spare concept as a means of mitigating the high risk of secondary drive failure on a parity RAID 5 array.  RAID 5 arrays protect only against the failure of a single disk within the array.  Once a single disk has failed the array is left with no form of parity and any additional drive failure results in the total loss of the array.  RAID 5 is chosen because it is very low cost for the given capacity and sacrifices reliability in order to achieve this cost effectiveness.   Because RAID 5 is therefore risky in comparison to other RAID options, such as RAID 6 or RAID 10, it is common to implement a warm spare in order to minimize the time that the array is left in a degraded state allowing the array to begin resilvering itself as quickly as possible.

So the takeaway here that is more relevant is that warm spares are generally used as a buffer against using less reliable RAID array types as a cost saving measure.  Warm spares are dramatically more common in RAID 5 arrays followed by RAID 6 arrays.  Both of which are chosen over RAID 10 due to cost for capacity, not for reliability or performance.  There is one case where the warm spare idea truly does make sense for added reliability, and that is in RAID 10 with a warm spare, but we will come to that.  Outside of that scenario I feel that warm spares make little sense in the real world.

We will start by examining RAID 1 with a warm spare.  RAID 1 consists of two drives, or more, in a mirror.  Adding a warm spare is nice in that if one of the mirrored pairs dies the warm spare will immediately begin mirroring the remaining drive and you will be protected again in short order.  That is wonderful.  Except for one minor flaw, instead of using a warm spare that same drive could have been added to the RAID 1 array all along where it would have been a tertiary mirror.  In this tertiary mirror capacity the drive would have added to the overall performance of the array giving a nearly fifty percent read performance boost with write performance staying level and providing instant protection in case of a drive failure rather than “as soon as it remirrors” protection.  Basically it would have been a true “hot spare” rather than a warm spare.  So without spending a penny more the system would have had better drive array performance and better reliability simply by having the extra drive in a hot “in the array” capacity rather than sitting warm and idle waiting for disaster to strike.

With RAID 5 we see an even more dramatic warning against the warm spare concept, here where it is more common than anywhere else.  RAID 5 is single parity RAID with the ability to rebuild, using the parity, any drive in the array that fails.  This is where the real problems begin.  Unlike in RAID 1 where a remirroring operation might be quite quick, a RAID 5 resilver (rebuild) has the potential to take quite a long time.  The warm spare will not assist in protecting the array until this resilver process completes successfully – this is commonly many hours and is easily days and possibly weeks or months depending on the size of the array and how busy the array is.  If we took that same warm spare drive and instead tasked it with being a member of the array with an additional parity stripe we would achieve RAID 6.  The same set of drives that we have for RAID 5 plus warm spare would create a RAID 6 array of the exact same capacity.  Again, like the RAID 1 example above, this would be much like having a hot spare, where the drive is participating in the array with live data rather than sitting idly by waiting for another drive to fail before kicking in to begin the process of taking over.  In this capacity the array degrades to a RAID 5 equivalent in case of a failure but without any rebuild time, so the additional drive is useful immediately rather than only after a possible very lengthy resilver process.  So for the same money, same capacity the choice of setting up the drives in RAID 6 rather than RAID 5 plus warm spare is a complete win.

We can continue this example with RAID 6 plus warm spare.  This one is a little less easy to define because in most RAID systems, except for the somewhat uncommon RAIDZ3 from ZFS, there is no triple parity system available one step above RAID 6 (imagine if there was a RAID 7, for example.)  If there were the exact argument made for RAID 5 plus warm spare would apply to RAID 6 plus warm spare.  In the majority of cases RAID 6 with a warm spare must justify itself against a RAID 10 array.  RAID 10 is more performant and far more reliable than a RAID 6 array but RAID 6 is generally chosen to save money in comparison to RAID 10.  But to offset RAID 6’s fragility warm spares are sometimes employed.  In some cases, such as a small five disk RAID 6 array with a warm spare, this is dollar for dollar equivalent to a six disk RAID 10 array without a warm spare.  In larger arrays the cost benefit of RAID 6 does become apparent but the larger the cost savings the larger the risk differential as parity RAID systems increase risk with array size much more quickly than do mirror based RAID systems like RAID 10.  Any money saved today is done at the risk of outage or data loss tomorrow.

Where a warm spare comes into play effectively is in a RAID 10 array where a warm spare rebuild is a mirror rebuild, like in RAID 1, which does not carry parity risks, where there is no logical extension RAID system above RAID 10 from which we are trying to save money by going with a more fragile system.  Here adding a warm spare may make sense for critical arrays because there is no more cost effective way to gain the same additional reliability.  However, RAID 10 is so reliable without a warm spare that any shop contemplating RAID 5 or RAID 6 with a warm spare would logically stop at simple RAID 10 having already surpassed the reliability they were considering settling for previously.  So only shops not considering those more fragile systems and looking for the most robust possible option would logically look to RAID 10 plus warm spare as their solution.

Just for technical accuracy, RAID 10 can be expanded for better read performance and dramatic improvement in reliability (but with a fifty percent cost increase) by moving to three disk RAID 1 mirrors in its RAID 0 stripe rather than standard two disk RAID 1 mirrors just like we showed in our RAID 1 example.  This is a level of reliability seldom sought in the real world but can exist and is an option.  Normally this is curtailed by drive count limitations in physical array chassis as well as competing poorly against building a completely separate secondary RAID 10 array in a different chassis and then mirroring these at a high level effectively created RAID 101 – which is the effective result of common, high end storage array clusters today.

Our second concern is that of “automated array destruction.”  This applies only to the parity RAID scenarios of RAID 5 and RAID 6 (or the rare RAID 2, RAID 3, RAID 4 and RAIDZ3.)  With the warm spare concept, the idea is that when a drive fails the warm spare is automatically and instantly swapped in by the array controller and the process of resilvering the array begins immediately.  If resilvering was a completely reliable process this would be obviouslyd highly welcomed.  The reality is, sadly, quite different.

During a resilver process a parity RAID array is at risk of Unrecoverable Read Errors (UREs) cropping up.  If a URE occurs in a single parity RAID resilver (that is RAID 2 –  5) then the resilvering process fails and the array is lost completely.  This is critical to understand because no additional drive has failed.  So if the warm spare had not been present then the resilvering would have not commenced and the data would still be intact and available – just not as quickly as usual and at the small risk of secondary drive failure.  URE rates are very high with today’s large drives and with large arrays the risks can become so high as to move from “possible” to “expected” during a standard resilvering operation.

So in many cases the warm spare itself might actually be the trigger for the loss of data rather than the savior of the data as expected.  An array that would have survived might be destroyed by the resilvering process before the human who manages it is even alerted to the first drive having failed.  Had a human been involved they could have, at the very least, taken the step to make a fresh backup of the array before kicking off the resilver knowing that the latest copy of the data would be available in case the resilver process was unsuccessful.  It would also allow the human to schedule when the resilver should begin, possibly waiting until business hours are over or the weekend has begun when the array is less likely to experience heavy load.

Dual and triple parity RAID (RAID 6 and RAIDZ3 respectively) share URE risks as well as they too are based on parity.  They mitigate this risk through the additional levels of parity and do so successfully for the most part.  The risk still exists, especially in very large RAID 6 arrays, but for the next several years the risks remain generally quite low for the majority of storage arrays until far larger spindle-based storage media is available on the market.

The biggest problem with parity RAID and the URE risk is that the driver towards parity RAID (willing to face additional data integrity risks in order to lower cost) is the same driver that introduces heightened URE risk (purchasing lower cost, non-enterprise SATA hard drives.)  Shops facing parity RAID generally do so with large, low cost SATA drives bringing two very dangerous factors together for an explosive combination.  Using non-parity RAID 1 or RAID 10 will completely eliminate the issue and using highly reliable enterprise SAS drives will drastically reduce the risk factor by an order of magnitude (not an expression, it is actually a change of one order of magnitude.)

Additionally during resilver operations it is possible for performance to degrade on parity systems so drastically as to equate to a long-term outage.  The resilver process, especially on large arrays, can be so intensive that end users cannot differentiate between a completely failed array and a resilvering array.  In fact, resilvering at its extreme can take so long and be so disruptive that the cost to the business can be higher than if the array had simply failed completely and a restore from backup had been done instead.  This resilver issue does not affect RAID 1 and RAID 10, again, because they are mirrored, not parity, RAID systems and their resilver process is trivial and the performance degradation of the system is minimal and short lived.  At its most extreme, a parity resilver could take weeks or months during which time the systems act as though they are offline – and at any point during this process there is the potential for the URE errors to arise as mentioned above which would end the resilver and force the restore from backup anyway.  (Typical resilvers do not take weeks but do take many hours and to take days is not at all uncommon.)

Our final overview can be broken down to the following (conventional term “hot spare” used again): RAID 10 without a “hot spare” is almost always a better choice than RAID 6 with a “hot spare.”  RAID 6 without a “hot spare” is always better than RAID 5 with a “hot spare.”  RAID 1 with additional mirror member is always better than RAID 1 with a “hot spare.”  So whatever RAID level with a hot spare you decide upon, simply move up one level of RAID reliability and drop the “hot spare” to maximize both performance and reliability for equal or nearly equal cost.

Warm spares, like parity RAID, had they day in the sun.  In fact it was when parity RAID still made sense for widespread use – when URE errors were unlikely and disk costs were high – that warm spare drives made sense as well.  They were well paired, when one made sense the other often did too.  What is often overlooked is that as parity RAID, especially RAID 5, has lost effectiveness it has pulled the warm spare along with it in unexpected ways.

When No Redundancy Is More Reliable – The Myth of Redundancy

Risk in a difficult concept and it requires a lot of training, thought and analysis to properly assess given scenarios.  Often, because risk assessments are so difficult, we substitute risk analysis with simply adding basic redundancy and assuming that we have appropriately mitigated risk.  But very often this is not the case.  The introduction of complexity or additional failure modes often accompany the addition of redundancy and these new forms of failure have the potential to add more risk than the added redundancy removes.  Storage systems are especially prone to these decision processes which is unfortunate as few, if any, systems are so susceptible to failure and more important to protect.

RAID is a great example of where a lack of holistic risk thinking can lead to some strange decision making.  If we look at a not uncommon scenario we will see where the goal of protecting against drive failure can actually lead to an increase in risk even when additional redundancy is applied.  In this scenario we will compare a twelve drive array consisting of twelve three terabyte SATA hard drives in a single array.  It is not uncommon to hear of people choosing RAID 5 for this scenario to get “maximum capacity and performance” while having “adequate protection against failure.”

The idea here is that RAID 5 protects against the loss of a single drive which can be replaced and the array will rebuild itself before a second drive fails.  That is great in theory, but the real risks of an array of this size, thirty six terabytes of drive capacity, come not from multiple drive failures as people generally suspect but from an inability to reliably rebuild the array after a single drive failure or from a failure of the array itself with no individual drives failing.  The risk of a second drive failing is low, not non-existent, but quite low.  Drives today are highly reliable. Once one drives fails it does increase the likelihood of a second drive failing, which is well documented, but I don’t want this risk to mislead us from looking at the true risks – the risk of a failed resilvering operation.

What happens that scares us during a RAID 5 resilver operation is that an unrecoverable read error (URE) can occur.  When it does the resilver operation halts and the array is left in a useless state – all data on the array is lost.  On common SATA drives the rate of URE is 10^14, or once every twelve terabytes of read operations.  That means that a six terabyte array being resilvered has a roughly fifty percent chance of hitting a URE and failing.  Fifty percent chance of failure is insanely high.  Imagine if your car had a fifty percent chance of the wheels falling off every time that you drove it.  So with a small (by today’s standards) six terabyte RAID 5 array using 10^14 URE SATA drives, if we were to lose a single drive, we have only a fifty percent chance that the array will recover assuming the drive is replaced immediately.  That doesn’t include the risk of a second drive failing, only the risk of a URE failure.  It also assumes that the drive is completely idle other than the resilver operation.  If the drives are busily being used for other tasks at the same time then the chances of something bad happening, either a URE or a second drive failure, begin to increase dramatically.

With a twelve terabyte array the chances of complete data loss during a resilver operation begin to approach one hundred percent – meaning that RAID 5 has no functionality whatsoever in that case.  There is always a chance of survival, but it is very low.  At six terabytes you can compare a resilver operation to a game of Russian roulette with one bullet and six chambers and you have to pull the trigger three times.  With twelve terabytes you have to pull it six times!  Those are not good odds.

But we are not talking about a twelve terabyte array.  We are talking about a thirty six terabyte array – which sounds large but this is a size that someone could easily have at home today, let alone in a business.  Every major server manufacturer, as well as nearly all low cost storage vendors, make sub $10,000 storage systems in this capacity range today.  Resilvering a RAID 5 array with a single drive failure on a thirty six terabyte array is like playing Russian roulette, one bullet, six chambers and pulling the trigger eighteen times!  Your data doesn’t stand much of a chance.  Add to that the incredible amount of time needed to resilver an array of that size and the risk of a second disk failing during that resilver window starts to become a rather significant threat.  I’ve seen estimates of resilver times climbing into weeks or months on some systems.  That is a long time to run without being able to lose another drive.  When we are talking hours or days the risks are pretty low, but still present.  When we are talking weeks or months of continuous abuse, as resilver operations are extremely drive intensive, the failure rates climb dramatically.

With an array of this size we can effectively assume that the loss of a single drive means the loss of the complete array leaving us with no drive failure protection at all.  Now if we look at a drive of the same or better performance with the same or better capacity under RAID 0, which also has no protection against drive loss, we need only use eleven of the same drives that we needed twelve of for our RAID 5 array.  What this means is that instead of twelve hard drives, each of which has a roughly three percent chance of annual failure, we have only eleven.  That alone makes our RAID 0 array more reliable as there are fewer drives to fail.  Not only do we have fewer drives but there is no need to write the parity block nor skip parity blocks when reading back lowering, ever so slightly, the mechanical wear and tear on the RAID 0 array for the same utilization giving it a very slight additional reliability edge.  The RAID 0 array of eleven drives will be identical in capacity to the twelve drive RAID 5 array but will have slightly better throughput and latency.  A win all around.  Plus the cost savings of not needing an additional drive.

So what we see here is that in large arrays (large in capacity, not in spindle count) that RAID 0 actually passes RAID 5 in certain scenarios.  When using common SATA drives this happens at capacities experienced even by power users at home and by many small businesses.  If we move to enterprise SATA drives or SAS drives then the capacity number where this occurs becomes very high and is not a concern today but will be in just a few years when drive capacities get larger still.  But this highlights how dangerous RAID 5 is in the sizes that we see today.  Everyone understands the incredible risks of RAID 0 but it can be difficult to put into perspective that RAID 5’s issues are so extreme that it might actually be less reliable than RAID 0.

That RAID 5 might be less reliable than RAID 0 in an array of this size based on resilver operations alone is just the beginning.  In a massive array like this the resilver time can take so long and exact such a toll on the drives that second drive failure starts to become a measurable risk as well.  And then there are additional risks caused by array controller errors that can utilize resilver algorithms to destroy an entire array even when no drive failure has occurred.  As RAID 0 (or RAID 1 or RAID 10) do not have resilver algorithms they do not suffer this additional risk.  These are hard risks to quantify but what is important is that they are additional risks that accumulate when using a more complex system when a simpler system, without the redundancy, was more reliable from the outset.

Now that we have established that RAID 5 can be less reliable than RAID 0 I will point out the obvious dangers of RAID 0.  RAID in general is used to mitigate the risk of a single, lone hard drive failing.  We all fear a single drive simply failing and all data being lost.  RAID 0, being a large stripe of drives without any form of redundancy, takes the risk of data loss of a single drive failing and multiplies it across a number of drives where any drive failing causes total loss of data to all drives.  So in our eleven disk example above, if any of the eleven disks fails all is lost.  It is clear to see where this is dramatically more dangerous than just using a single drive, all alone.

What I am trying to point out here is that redundancy does not mean reliability.  Just because something is redundant, like RAID 5, provides no guarantee that it will always be more reliable than something that is not redundant.

My favourite analogy here is to look at houses in a tornado.  In one scenario we build a house of brick and mortar.  On the second scenario we build two redundant house, east out of straw (our builders are pigs, apparently.)  When the tornado (or big bad wolf) comes along which is more likely to leave us with a standing house?  Clearing one brick and mortar house has some significant reliability advantages over redundant straw houses.  Redundancy didn’t matter, reliability mattered in the end.

Redundancy is often misleading because it is easy to quantify but hard to qualify.  Redundancy is a black or white question: Is it redundant?  Yes or no.  Simple.  Reliability is not so simple.  Reliability is about failure rates and likelihoods.  It is about statistics and analysis.  As it is hard to quantify reliability in a meaningful way, especially when selling a project to the business people, redundancy often becomes a simple substitute for this complex concept.

The concept of using redundancy to misdirect questions of reliability also ends up applying to subsystems in very convoluted ways.  Instead of making a “system” redundant it has become common to make a highly reliable, and low cost, subsystem redundant and treat subsystem redundancy as applying to the whole system.  The most common example of this is RAID controllers in SAN products.  Rather than having a redundant SAN (meaning two SANs) manufacturers will often make that one component not often redundant in normal servers redundant  and then calling the SAN redundant – meaning a SAN that contains redundancy, which is not at all the same thing.

A good analogy here would be to compare having redundant cars meaning two complete, working cars and having a single car with a spare water pump in the trunk in case the main one fails.  Clearly, a spare water pump is not a bad thing.  But it is also a trivial amount of protection against car failure compared to having a second car ready to go.  In one case the entire system is redundant, including the chassis.  In the other we are making just one, highly reliable component redundant inside the chassis.  It’s not even on par with having a spare tire which, at least, is a car component with a higher likelihood of failure.

Just like the myth of RAID 5 reliability and system/subsystem reliability, shared storage technologies like SANs and NAS often get treated in the same way, especially in regards to virtualization.  There is a common scenario where a virtualization project is undertaken and people instinctively panic because a single virtualization host represents a single point of failure where, if it fails, many systems will all fail at once.

Using the term “single point of failure” causes a panic feeling and is a great means of steering a conversation.  But a SPOF, as we like to call it, while something we like to remove when possible may not be the end of the world.  Think about our brick house.  It is a SPOF.  Our two houses of straw are not.  Yet a single breeze takes out our redundant solutions faster than our reliable SPOF.  Looking for SPOFs is a great way to find points of fragility in a system, but do not feel that every SPOF must be made redundant in every scenario.  Most businesses will find their best value having many SPOFs in place.  Our real goal is reliability at appropriate cost, redundancy, as we have seen, is no substitute for reliability, it is simply a tool that we can use to achieve reliability.

The theory that many people follow when virtualizing is that they take their virtualization host and say “This host is a SPOF, so I need to have two of them and use High Availability features to allow for transparent failover!”  This is spurred by the leading virtualization vendor making their money firstly by selling expensive HA add on products and secondly by being owned by a large storage vendor – so selling unnecessary or even dangerous additional shared storage is a big monetary win for them and could easily be the reason that they have championed the virtualization space from the beginning.  Redundant virtualization hosts with shared storage sounds great but can be extremely misguided for several reasons.

The first reason is that removing the initial SPOF, the virtualization host, is replaced with a new SPOF, the shared storage.  This accomplishes nothing.  Assuming that we are using comparable quality servers and shared storage all we’ve done is move where the risk is, not change how big it is.  The likelihood of the storage system failing is roughly equal to the likelihood of the original server failing.  But in addition to shuffling the SPOF around like in a shell game we’ve also done something far, far worse – we have introduced chained or cascading failure dependencies.

In our original scenario we had a single server.  If the server stayed working we are good, if it failed we were not.  Simple.  Now we have two virtualization hosts, a single storage server (SAN, NAS, whatever) and a network connecting them together.  We have already determined that the risk of the shared storage failing is approximately equal to our total system risk in the original scenario.  But now we have the additional dependencies of the network and the two front end virtualization nodes.  Each of these components is more reliable than the fragile shared storage (anything with mechanical drives is going to be fragile) but that they are lower risk is not the issue, the issue is that the risks are combinatorial.

If any of these three components (storage, network or the front end nodes) fail then everything fails.  The solution to this is to make the shared storage redundant on its own and to make the network redundant on its own.  With enough work we can overcome the fragility and risk that we introduced by adding shared storage but the shared storage on its own is not a form of risk mitigation but is a risk itself which must be mitigated.  The spiral of complexity begins and the cost associated with bringing this new system up on par with the reliability of the original, single server system can be astronomic.

Now that we have all of this redundancy we have one more risk to worry about.  Managing all of this redundancy, all of these moving parts, requires a lot more knowledge, skill and preparation than does managing a simple, single server.  We have moved from a simple solution to a very complex one.  In my own anecdotal experience the real dangers of solutions like this come not from the hardware failing but from human error.  Not only has little been done to avoid human error causing this new system to fail but we’ve added countless points where a human might accidentally bring the entire system, redundancy and all, right down.  I’ve seen it first hand; I’ve heard the horror stories.  The more complex the system the more likely a human is going to accidentally break everything.

It is critical that as IT professionals that we step back and look at complete systems and consider reliability and risk and think of redundancy simply as a tool to use in the pursuit of reliability.  Redundancy itself is not a panacea.  Neither is simplicity.  Reliability is a complex problem to tackle.  Avoiding simplistic replacements is an important first step in moving from covering up reliability issues to facing and solving them.


Choosing an Open Storage Operating System

It is becoming increasingly common to forgo traditional, proprietary storage devices, both NAS and SAN, and instead using off the shelf hardware and installing a storage operating system on it for, what many call, “do it yourself” storage servers.  This, of course, is a misnomer since no one calls a normal file server “do it yourself” just because you installed Windows yourself.  Storage has a lot of myth and legend swirling around it and people often panic when the they think of installing Windows and calling it NAS rather than calling it a file server.  So, if it makes you feel better, use terms like file server or storage server rather than NAS and SAN – problem solved.  This is a part of the “open storage” movement – moving storage systems from proprietary to standard.

Choosing the right operating system for a storage server is important and not always that easy.  I work extensively in this space and people often ask me what I recommend and the recommendations vary, based on scenario, and often seem confusing.  But the factors are actually relatively easy, if you just know the limitations that create the choices and paths in the decision tree.

Before choosing an OS we must stop and consider what our needs are going to be.  Some areas that need to be considered are: capacity, performance, ease of administration, budget, connection technology, cost and clustering.  There are two main categories of systems that we will consider as well, standard operating system or storage appliance operating system.  The standard operating systems are Windows, Linux, Solaris and FreeBSD.  The storage appliance operating systems are FreeNAS, OpenFiler and NexentaStor.  There are others in both categories but these are the main players currently.

The first decision to be made is whether or not you or your organization is comfortable supporting a normal operating system operating in a storage server role.  If you are looking at NAS then simply ask yourself if you could administer a file server.  Administrating a block storage server (SAN) is a little more complex or, at least, unusual, so this might induce a small amount of concern but is really in line with other administration tasks.  If the answer is yes, that using normal operating system tools and interfaces is acceptable to you, then simply rule out the “appliance” category right away.  The appliance approach adds complexity and slows development and support cycles, so unless necessary is undesirable.

Storage appliance operating systems exist only to provide a pre-packaged, “easy to use” view into running a storage server.  In concept this is nice, but there are real problems with this method.  The biggest problems come from the packaging process which pulls you a step away from the enterprise OS vendors themselves making your system more fragile, further behind in updates and features and less secure than the traditional OS counterparts.  It also leaves you at the mercy of a very small company for OEM-level support when something goes wrong rather than with a large enterprise vendor with a massive user base and community.  The appliancization process also strips features and options from the systems by necessity.  In the end, you lose.

Appliances are nice because you get a convenient web interface from which “anyone” can administer your storage.  At least in theory.  But in reality there are two concerns.  The first is that there is always a need to drop into the operating system itself and fix things every once in a while.  Having the custom web interface of the appliance makes this dramatically harder than normal so at the time when you most need the appliance nature of the system is when you do not have it.  The second is that making something as critical as storage available for “anyone” to work on is a terrifying thought.  There are few pieces of your infrastructure where you want more experience, planning and care taken than in storage.  Making the system harder to use is not always a bad thing.

If you are in need of the appliance system then primarily you are looking at FreeNAS and OpenFiler.  NexentaStor offers a compelling product but it is not available in a free version and the cost can be onerous.  The freely downloadable version appears to be free for the first 18TB of raw storage but the license states otherwise making this rarely the popular choice.  (The cost of NexentaStor is high enough that purchasing a fully supported Solaris system would be less costly and provides full support from the original vendor rather than Nexenta which is essentially repackaging old versions of Solaris and ZFS.  More modern code and updates are available less expensively from the original source.)

FreeNAS, outside of clustering, is the storage platform of choice in an appliancized package.  It has the much touted ZFS filesystem which gives it flexibility and ease of use lacking in OpenFiler and other Linux-based alternatives.  It also has a working iSCSI implementation so you can use FreeNAS safely as either a NAS or a SAN.  Support for FreeNAS appears to be increasing with new developments being made regularly and features being retained.  FreeNAS offers a large range of features and supported protocols.  It is believed that clustering will be coming to FreeNAS in the future as well as this has recently been added to the underlying FreeBSD operating system.  If so, FreeNAS will completely eliminate the need for OpenFiler in the marketplace.  FreeNAS is completely free.

OpenFiler lacks a reliable iSCSI SAN implementation (unless you pay a fortune to have that part of the system replaced with a working component ) and is far more out of date than its competitors but does offer full block-level real-time replication allowing it to operate in a clustered mode for reliability .  The issue here being that the handy web interface of the NAS appliance does not address this scenario and if you want to do this you will need to get your hands dirty on the command line, very dirty indeed.  This is expert level stuff and anyone capable of even considering a project to make OpenFiler into a reliable cluster will be just as comfortable, and likely far more comfortable, building the entire cluster from scratch on their Linux distribution of choice.  OpenFiler is built on the rather unpopular, and now completely discontinued, rPath Linux using the Conary packaging system both which are niche players, to say the least, in the Linux world.  You’ll find little rPath support from other administrators and many packages and features that you may wish access to are unavailable.  OpenFiler’s singular advantage of any significance is the availability of DRBD for clustering, which as stated above, in nonsensical.  Support for OpenFiler appears to be waning with new features being non-existant and, in fact, key features like the AFP have been dropped rather than new features having been added.  OpenFiler is free but key features, like reliable iSCSI, are not.  Recent reports from OpenFiler users are that even non-iSCSI storage has become unstable in the latest release and losing data is a regular occurrence.  OpenFiler remains very popular in the mindshare of this industry segment but should be avoided completely.

If you do not need to have your storage operating system appliancized then you are left with more and better choices, but a far more complex decision tree.    Unlike the appliance OS market which is filled with potholes (NexentaStor has surprise costs, OpenFiler appears to support iSCSI but causes data loss, features get removed from new versions) all four operating systems mentioned here are extremely robust and feature rich.  Three of them have OEM vendor support which can be a major deciding factor and all have great third party support options far broader than what is available for the appliance market.

The first decision is whether or not Windows only features, notably NTFS ACLs, are needed.  It is common for new NAS users to be surprised when the SMB protocol does not provide all of the granular filesystem control that they are used to in Windows.  This is because those controls are actually handled by the filesystem, not the network protocol, and Windows alone provides these via NTFS.  So if that granular Windows file control is needed, Windows is your only option.

The other three entrants, Linux, Solaris and FreeBSD, all share basic capabilities with the notable exception of clustering.  All have good software RAID, all have powerful and robust filesystems, all have powerful logical volume management and all provide a variety of NAS and SAN connection options.  Many versions of Linux and FreeBSD are available completely freely.  Solaris, while free for testing, is not available for free for production use.

The biggest differentiator between these three OS options is clustering.  Linux has had DRBD for a long time now and this is a robust filesystem clustering technology.  FreeBSD has recently (as of 9.0) added HAST to serve the same purpose.  So, in theory, FreeBSD has the same clustering options as Linux but this is much newer and much less well known.  Solaris lacks filesystem clustering in the base OS and requires commercial add-ons to handle this at this time.

Solaris and FreeBSD share the powerful and battle tested ZFS filesystem.  ZFS is extremely powerful and flexible and has long been the key selling point of these platforms.  Linux’ support for filesystems is more convoluted.  Nearly any Linux distribution (we care principally about RHEL/CentOS, Oracle Unbreakable Linux, Suse/OpenSuse and Ubuntu here) supports EXT4 which is powerful and fast but lacks some of the really nice ZFS features.  However, Linux is rapidly adopting BtrFS which is very competitive with ZFS but is nascent and currently only available in Suse and Oracle Linux distros.  We expect to see it from the others soon for production use but at this time it is still experimental.

Outside of clustering, likely the choice of OS of these three will come down primarily to experience and comfort.  Solaris is generally known for providing the best throughput and FreeBSD the worst.  But all three are quite close.  Once BtrFS is widely available and stable on Linux, Linux will likely become the de facto choice as it has been in the past.

Without external influence, my recommendation for storage platform are FreeBSD and then Linux with Solaris eliminated on the basis that rarely is anyone looking for commercial support and so it is ruled out automatically.  This is based almost entirely on the availability of Copy-on-Write filesystems and assuming no clustering which is not common.  If clustering is needed then Linux first then FreeBSD and Solaris is ruled out, again.

Linux and FreeBSD are rapidly approaching each other in functionality.  As BtrFS matures on Linux and HAST matures on FreeBSD they seem to be meeting in the middle with the choice being little more than a toss up.

There is no single, simple answer.  Choosing a storage OS is all about balancing myriad factors from performance, resources, features, support, stability, etc.  There are a few factors that can be used to rule out many contenders and knowing these hard delimiters is key.  Knowing exactly how you plan to use the system and what factors are important to you are important in weeding through the available options.

Even once you pick a platform there are many decisions to make.  Some platforms include multiple file systems.  There is SAN and NAS.  There are multiple SAN and NAS protocols.  There is network bonding (or teaming, the Windows world.)  There is Multipathing.  There are snapshots, volumes, RAID.  The list goes on and on.