[OpenIndiana-discuss] Raid type selection for large # of ssds

Roy Sigurd Karlsbakk roy at karlsbakk.net
Mon Oct 8 16:26:48 UTC 2012

> I feel bad asking this question because I generally know what raid
> type to pick.
> I am about to configure 24 256gig ssd drives in a ZFS/Comstar
> deployment. This will serve as the datastore for a vmware deployment.
> Does anyone know what raid level would be best. I know the workload
> will determine alot, but obviously there is varying workload across a
> vmware environment. Since we are talking about ssds I dont see a
> particular reason to not create 1 big zfs pool, with the exception
> that I know people generally try to keep the drive count from getting
> out of control. Raid 10 seems like a waste of space with little
> benefit in performance in this case. i am leaning towards raid z2 but
> wanted to get everyones input.
> The datastore will host a fileserver, and exchange server for about 50
> users. The environment is all 10g and they have solid states in all
> desktops so essentially that is the reason for such a large SSD
> deployment for a small # of users.
> There seems to be varying opinions, especially when you factor in
> trying to keep writes low for ssds.

I can only share my experience with spinning rust. As others have said, I'd recommend against a single VDEV with all 24 drives. The chance of two or three dying at once is rather high with that number of drives. SSDs die too. A choice of 3x8 in RAIDz2 seems reasonable, or perhaps 2x7+1x8+spare (I know, it's uneven, but not that much, and a spare is a good thing). That'll give you IOPS comparable with 3x IOPS of a single drive, which should be pretty good with most SSDs.

Also, keep in mind the problems with certain (or most?) SATA units connected to a SAS expander. I've seen pretty bad things happen with WD2001FASS drives in such a configuration (we had to replace about 160 drives and replace them with hitachis to solve that problem - not too much data was lost, though, thanks to pure luck).

- What sort of SSDs are these btw?

Vennlige hilsener / Best regards

Roy Sigurd Karlsbakk
(+47) 98013356
roy at karlsbakk.net
GPG Public key: http://karlsbakk.net/roysigurdkarlsbakk.pubkey.txt
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