markok
Contributor
Contributor

Write cache for ESX local disks

There are many articles and discussions about VM's virtual disk performance under the ESX server, but how is ESX itself performing if you have or don't have any write cache on your storage controller? Do I really need any write cache when only ESX itself is using the local disks (mirror) and all the rest is in SAN storage?

Also another question related to cache and disk controllers. If I have a storage controller with cache but I don't have battery to back it up (like Dell CERC6/i controller), what might be lost if power failure comes and there is something in cache waiting to be written? When comparing the ESX and ESXi, there shouldn't be any vital information waiting to be written.

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4 Replies
jhanekom
Virtuoso
Virtuoso

I think you're right in that there's not much of a performance requirement for the ESX OS disks. It certainly can't harm to add battery-backed write-cache, though, and in most instances it won't cost a lot of money.

Most controllers will disable write-caching if there is no battery to back up the cache, leaving only read-caching enabled.

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kjb007
Immortal
Immortal

Depending on what type of caching you are doing (write-back or write-through), there is a potential of data loss if you don't have a battery backup for your storage controller. If you haved write-through, then you'll be fine as the transaction will not be considered complete until data is written to disk, as opposed to the other, which returns success immediately after writing to cache.

It will generally be better to have a battery backup, but you can always use write-through to avoid the data loss. As mentioned, you will probably not notice much of a performance difference, unless you force high I/O, which ESX itself will not do if you're storing your vm's on the SAN.

-KjB

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB
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markok
Contributor
Contributor

Our hardware specs includes the battery backed write cache in the storage controller, but I was just thinking if we still need if for VMware environment. But like said, the price between non-cache and cache version is so little that there's no point to leave it out. Any opinions what data (I mean ESX's own local data) might be lost if write cache is enabled and no battery is available? Any virtual machine related data or is it only ESX log's etc. what is lost?

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kjb007
Immortal
Immortal

Most likely, you will not lose any real data. Since the VMs are stored elsewhere, that won't be an issue. There is a possibility, that if you were making changes to ESX config, then there could be a potential for file corruption, or just no config updates. If you lost power and your server crashed anyway, then you are already facing that risk, and in most cases, you will still be ok.

-KjB

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB