Death to false myths: The type of virtual disk used determines your performance

I was reading Eric Sloof’s article last week about how the disk type will impact your performance. First of all, let me be clear that I am not trying to bash Eric here… I think Eric has a valid point, well at least in some cases and let me explain why.

On his quest of determining if the virtual disk type determines performance Eric tested the following on his environment:

• Thin
• Lazy zero thick
• Eager zero thick

These are the three disk types you can choose from when creating a new virtual disk. The difference between them is simple. Thick are fully allocated virtual disk files. Lazy zero means that the disk is not zeroed out yet, eager zero means the full disk is zeroed out during the provisioning process. Thin, well I guess you know what it means… not fully allocated and not also not zeroed. This also implies that in the case of “thin” and “lazy zero thick” something needs to happen when a new “block” is accessed. This is what Eric showed in his test. But is that relevant?

First of all, the test was conducted with an Iomega PX6-300d. One thing to point out here  is that this device isn’t VAAI capable and limited from a performance perspective due to the CPU power and limited set of spindles. The lack of VAAI however impacts the outcome the most. This, in my opinion means, that the outcome cannot really be used by those who have VAAI capable arrays. The outcome would be different when one of the following two (or both) VAAI primitives are supported and used by the array:

• Write Same aka “Zero Out”
• ATS aka “Hardware Offloaded Locking”

Cormac Hogan wrote an excellent article on this topic and an excellent white paper, if you want to know more about ATS and Write Same make sure to read them.

Secondly, it is important to realize that most applications don’t have the same write pattern as the benchmarking tool used by Eric. In his situation the tool basically fills up an X amount of blocks sequentially to determine the max performance. Only if you do fill up a disk at once, or very large unwritten sections, you potentially could see a similar result. Let me emphasize that, could potentially.

I guess this myth was once a fact, back in 2009 a white paper was released about Thin/Thick disks. In this paper they demonstrate the difference between thin, lazy zero and eager zero thick… yes they do proof there is a difference but this was pre-VAAI. Now if you look at another example, a nice extreme example, which is a performance test done by my friends of Pure Storage you will notice there is absolutely no difference. This is an extreme example considering it’s an all-flash VAAI based storage system, nevertheless it proofs a point. But not just all-flash arrays see a huge improvement, take a look at this article by Derek Seaman about 3Par’s “write same” (zero’ing) implementation, I am pretty sure that in his environment he would also not see the huge discrepancy Eric witnessed.

I am not going to dispel this myth as it is a case of “it depends”. It depends on the type of array used, and for instance how VAAI was implemented as that could make difference. In most cases however it is safe to say that the performance difference will not be big, if noticeable at all during normal usage. I am not even discussing all the operational implications of using eager-zero-thick… (as Frank Denneman will respond soon to this blog post has a nice article about that. )