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Has anyone experimented with running a virtual RAID0 inside a VM? For example, two virtual disks in a software RAID0 configuration, rather than a single virtual disk.

Specifically, I'm wondering if read performance would increase, or if the overhead of having two virtual disks would negate the effect?

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2 Answers 2

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I wouldn't expect any improvement unless your virtual disks were on physically separate media/spindles on the host system -- Having two virtual disks in RAID0 that are mapped to one physical disk on the host will probably just slow things down as the physical drive will have to seek back and forth between the virtual disk images in order to accommodate the striping.
Even with physically separated underlying datastores I think the VM overhead will trump any performance gain.

Typically I don't build RAIDs in my VM systems -- The underlying host has RAID for redundancy (with the RAID level (1/5/6) chosen based on the performance I need from that VM environment), and the individual VMs just ride on top of that with what they see as one single big (virtual) disk.

Your mileage may vary, and there are probably good cases for presenting two virtual disks to some VMs, but I'm pretty confident in saying that performance isn't one of them.

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I was thinking the same thing. But, my ESX host has a couple dozen drives in it, if I have some time in the future I'll give it a shot and do some benchmarks. Thanks –  Seth Jan 27 '11 at 19:50
    
It would be interesting to see benchmarks if you have the time - My gut tells me you'd get better performance setting up RAID-0 or -10 on the underlying drives (and/or splitting them up into multiple arrays to manually distribute load like you do for a database server's log spindle) –  voretaq7 Jan 27 '11 at 19:55

There may be some minor, fractional percentage point improvements in high I/O applications through parallelizing the I/O elevator over multiple virtual LUNs, but that's going to be pretty small. The performance gains are not enough to overcome the effort of setting it up in the first place. Not worth it.

Presenting multiple virtual-disks based on different storage arrays actually will improve performance, for the same reasons doing so on physical does. If the presented-storage is also RAIDed appropriately, the chief risk of R0 is largely negated; you'd need an entire array failure to lose storage instead of a single disk failure.

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The more I think about this, the more it makes sense - you're effectively building a RAID N+0, with virtual RAID 0 over the physical raid N arrays. The caveat is that you'd need to be very sure that your disk performance was a significant bottleneck before deciding to use this vs. upgrading some other part of the VM host. –  Andrew Jan 21 '11 at 1:10
    
@Andrew BINGO. Knowing you're I/O bound on a single disk array is the only reason you'd do that. –  sysadmin1138 Jan 21 '11 at 1:38

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