Thoughts on Thin Provisioning

I finally decided to put all my thoughts down on the topic of thin provisioning.  I wrestled with this post for a while because some of what I say is going to go kinda-sorta against a large push in the industry towards thin provisioning.  This is not a new push; it has been happening for years now.  This post may even be a year or two too late…

I am not anti-thin – I am just not 100% pro-thin.  I think there are serious questions that need to be addressed and answered before jumping on board with thin provisioning.  And most of these are relatively non-technical; the real issue is operational.

Give me a chance before you throw the rocks.

What is Thin Provisioning?

First let’s talk about what thin provisioning is, for those readers who may not know.  I feel like this is a pretty well known and straightforward concept so I’m not going to spend a ton of time on it.  Thin provisioning at its core is the idea of provisioning storage space “on demand.”

Before thin provisioning a storage administrator would have some pool of storage resources which gave some amount of capacity.  This could be simply a RAID set or even an actual pooling mechanism like Storage Pools on VNX.  A request for capacity would come in and they would “thick provision” capacity out of the pool.  The result would mean that the requested capacity would be reserved from the pooled capacity and be unavailable for use…except obviously for whatever purpose it was provisioned for.  So for example if I had 1000GB and you requested a 100GB LUN, my remaining pool space would be 900GB.  I could use the 900GB for whatever I wanted but couldn’t encroach into your 100GB space – that was yours and yours alone.  This is a thick provisioned LUN.

Of course back then it wasn’t “thick provisioning,” it was just “provisioning” until thin came along! With thin provisioning, after the request is completed and you’ve got your LUN, the pool is still at 1000GB (or somewhere very close to it due to metadata allocations which are beyond the scope of this post).  I have given you a 100GB LUN out of my 1000GB pool and still I have 1000GB available.  Remember that as soon as you get this 100GB LUN, you will usually put a file system on it and then it will appear empty.  This emptyness is the reason that the 100GB LUN doesn’t take up any space…there isn’t really any data on it until you put it there.

Essentially the thin LUN is going to take up no space until you start putting stuff into it.  If you put 10GB of data into the LUN, then it will take up 10GB on the back side.  My pool will now show 990GB free.  You should have a couple of indicators on the array like allocated or subscribed or committed and consumed or used.  Allocated/subscribed/committed is typically how much you as the storage administrator have created in the pool.  Consumed or used is how much the servers themselves have eaten up.

What follows are, in no particular order, some things to keep in mind when thin provisioning.

Communication between sysadmin and storage admin

This seems like a no-brainer but a discussion needs to happen between the storage admins providing the storage and the sysadmins who are consuming it.  If a sysadmin is given some space, they typically see this as space they can use for whatever they want.  If they need a dumping ground for a big ISO, they can use the SAN attached LUN with 1TB of free space on it.  Essentially they will likely feel that space you’ve allocated is theirs to do whatever they want with.  This especially makes sense if they’ve been using local storage for years.  If they can see disk space on their server, they can use it as they please.  It is “their” storage!

You need to have this conversation so that sysadmins understand activities and actions that are “thin hostile.”  A thin hostile action is one that effectively nullifies the benefit of thin provisioning by eating up space from day 1.  An example of a thin hostile action would be hard formatting the space a 500GB database will use up front, before it is actually in use.  Another example of a thin hostile action would be to do a block level zero formatting of space, like Eager Zero Thick on ESX.  And obviously using excess free space on a LUN for a file dumping ground is extremely thin hostile!

Another area of concern here is deduplication.  If you are using post-process deduplication, and you have thin provisioned storage, your sysadmins need to be aware of this when it comes to actions that would overwrite a significant amount of data.  You may dedupe their data space by 90%, but if they come in and overwrite everything it can balloon quickly.

The more your colleagues know about how their actions can affect the underlying storage, the less time you will spend fire fighting.  Good for them, good for you.  You are partners, not opponents!

Oversubscription & Monitoring

With thin provisioning, because no actual reservation happens on disk, you can provision as much storage as you want out of as small a pool as you want.  When you exceed the physical media, you are “oversubscribing” (or overcommitting, or overprovisioning, or…).  For instance, with your 1000GB you could provision 2000GB of storage.  In this case you would be 100% oversubscribed.  You don’t have issues as long as the total used or consumed portion is less than 1000GB.

There are a lot of really appealing reasons for doing this.  Most of the time people ask for more storage than they really need…and if it goes through several “layers” of decision makers, that might amplify greatly.  Most of the time people don’t need all of the storage they asked for right off the bat.  Sometimes people ask for storage and either never use it or wait a long time to use it.  The important thing to never forget is that from the sysadmin’s perspective, that is space you guaranteed them!  Every last byte.

Oversubscription is a powerful tool, but you must be careful about it.  Essentially this is a risk-reward proposition: the more people you promise storage to, the more you can leverage your storage array, but the more you risk that they will actually use it.  If you’ve given out 200% of your available storage, that may be a scary situation when a couple of your users decide to make good on the promise of space you made to them.  I’ve seen environments with as much as 400% oversubscription.  That’s a very dangerous gamble.

Thin provisioning itself doesn’t provide much benefit unless you choose to oversubscribe.  You should make a decision on how much you feel comfortable oversubscribing.  Maybe you don’t feel comfortable at all (if so, are you better off thick?).  Maybe 125% is good for you.  Maybe 150%.  Nobody can make this decision for you because it hinges on too many internal factors.  The important thing here is to establish boundaries up front.  What is that magic number?  What happens if you approach it?

Monitoring goes hand in hand with this.  If you monitor your environment by waiting for users to email that systems are down, oversubscribing is probably not for you.  You need to have a firm understanding of how much you’ve handed out and how much is being used.  Again, establish thresholds, establish an action plan for exceeding them, and monitor them.

Establishing and sticking with thresholds like this really helps speed up and simplify decision making, and makes it very easy to measure success.  You can always re-evaluate the thresholds if you feel like they are too low or too high.

Also make sure your sysadmins are aware of whether you are oversubscribed or not, and what that means to them.  If they are planning on a massive expansion of data, maybe they can check with you first.  Maybe they requested storage for a project and waited 6 months for it to get off the ground – again they can check with you to make sure all is well before they start in on it.  These situations are not about dictating terms, but more about education.  Many other resources in your environment are likely oversubscribed.  Your network is probably oversubscribed.  If a sysadmin in the data center decided to suddenly multicast an image to a ton of servers on a main network line, you’d probably have some serious problems.  You probably didn’t design your network to handle that kind of network traffic (and if you did you probably wasted a lot of money).  Your sysadmins likely understand the potential DDoS effect this would generate, and will avoid it.  Nobody likes pain.

“Runway” to Purchase New Storage

Remember with thin provisioning you are generally overallocating and then monitoring (you are monitoring, aren’t you?) usage.  At some point you may need to buy more storage.

If you wait till you are out of storage, that’s no good right?  You have a 100% consumed pool, with a bunch of attached hosts that are thinking they have a lot more storage to run through.  If you have oversubscribed a pool of storage and it hits 100%, it is going to be a terrible, horrible, no good, very bad day for you and everyone around you.  At a minimum new writes to anything in that pool will be denied, effectively turning your storage read-only.  At a maximum, the entire pool (and everything in it) may go offline, or you may experience a variety of fun data corruptions.

So, you don’t want that.  Instead you need to figure out when you will order new storage.  This will depend on things like:

  • How fast is your storage use growing?
  • How many new projects are you implementing?
  • How long does it take you to purchase new storage?

The last point is sometimes not considered before it is too late.  When you need more storage you have to first figure out exactly what you need, then you need to spec it, then you need a quote, the quote needs approval, then purchasing, then shipping, then it needs to be racked/stacked, then implemented.  How long does this process last for your organization?  Again nobody can answer this but you.  If your organization has a fast turn around time, maybe you can afford to wait till 80% full or more.  But if you are very sluggish, you might need to start that process at 60% or less.

Another thing to consider is if you are a sluggish organization, you may save money by thick provisioning.  Consider that you may need 15TB of storage in 2 years.  Instead you buy 10TB of storage right off the bat with a 50% threshold.  As soon as you hit 5TB of storage used you buy another 10TB to put you at 20.  Then when you hit 10 you buy another 10TB to put you at 30.  Finally at 15TB you purchase again and hit 40TB.  If you had bought 20 to begin with and gone thick, you would have never needed to buy anything else.  This situation is probably uncommon but I wanted to mention it as a thought exercise.  Think about how the purchasing process will impact the benefit you are trying to leverage from thin provisioning.

Performance Implications

Simply – ask your vendor whether thin storage has any performance difference over thick.  The answer with most storage arrays (where you have an actual choice between thick and thin) is yes.  Most of the time this is a negligible difference, and sometimes the difference is only in the initial allocation – that is to say, the first write to a particular LBA/block/extent/whatever.  But again, ask.  And test to make sure your apps are happy on thin LUNs.

Feature Implications

Thin provisioning may have feature implications on your storage system.

Sometimes thin provisioning enables features.  On a VMAX, thin provisioning enables pooling of a large number of disks.  On a VNX thin provisioning is required for deduplication and VNX Snapshots.

And sometimes thin provisioning either disables or is not recommended with certain features.  On a VNX thin LUNs are not recommended for use as File OE LUNs, though you can still do thin file systems on top of thick LUNs.

Ask what impact thin vs thick will have on array features – even ones you may not be planning to use at this very second.

Thin on Thin

Finally, in virtualized environments, in general you will want to avoid “thin on thin.”  This is a thin datastore created on a thin LUN.  The reason is that you tend to lose a static point of reference for how much capacity you are overprovisioning.  And if your virtualization team doesn’t communicate too well with the storage team, they could be unknowingly crafting a time bomb in your environment.

Your storage team might have decided they are comfortable with a 200% oversubscription level, and your virt team may have made this same decision.  This will potentially overallocate your storage by 400%!  Each team is sticking to their game plan, but without knowing and monitoring the other folks they will never see the train coming.

You can get away with thin on thin if you have excellent monitoring, or if your storage and virt admins are one and the same (which is common these days).  But my recommendation still continues to be thick VMs on thin datastores.  You can create as many thin datastores as you want, up to system limits, and then create lazy zeroed thick VMs on top of them.

Edit: this recommendation assumes that you are either required or compelled to use thin storage.  Thin VMs on thick storage are just as effective, but sometimes you won’t have a choice in this matter.  The real point is keeping one side or the other thick gives you a better point of reference for the amount of overprovisioning.


Hopefully this provided some value in the form of thought processes around thin provisioning.  Again, I am not anti-thin; I think it has great potential in some environments.  However, I do think it needs to be carefully considered and thought through when it sometimes seems to be sold as a “just thin provision, it will save you money” concept.  It really needs to be fleshed out differently for every organization, and if you take the time to do this you will not only better leverage your investment, but you can avoid some potentially serious pain in the future.

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