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I am considering implementing an iSCSI shared storage array using SATA SSDs instead of the 15k RPM SAS drives we normally purchase.

We normally use RAID 10 because of spindle contention with the random IO produced by virtualized workloads.

I was wondering if we could switch to RAID 5 or RAID 6 to have more usuable space now that spindle contention is less of an issue. A question in my mind is how much overhead there is from the controller calculating parity. I am aware that this configuration will not allow TRIM to function.

Our current workloads are running on a Dell H800 with a 24 bay external enclosure.

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Garbage Collection and TRIM are two entirely separate concepts. GC refers to how the drive handles pages that have been overwritten, while TRIM is how the OS lets the drive know that the data has been deleted. – DerfK Nov 25 '12 at 2:27
Thanks for the clarification. After reading up on this I'm doubtful that even if we weren't considering RAID it would be difficult for TRIM to function when the LVM block devices will be exported via iSCSI rather than at a filesystem level. – root Nov 25 '12 at 4:28
Are the sata ssds hot pluggable? – 3molo Nov 25 '12 at 9:31
I'm pretty sure anything with a SATA or SAS interface is hot-pluggable now. At least, I've never found a device I couldn't add/remove and then rescan the bus. – root Nov 26 '12 at 0:51
@3molo hot plugging is basically mandatory part of SATA specification since version 2.0 and all versions of SAS. – Hubert Kario Nov 26 '12 at 5:01
up vote 1 down vote accepted

Modification of single block will still require reading whole stripe from the disk and saving it again, or recalculating the parity block (depending on RAID implementation) and saving it to disk. Because SSDs have less problems performing random accesses it won't suffer as much performance degradation as similar array with HDDs but it won't be negligible.

The other problem you may run into is the performance of the controller itself. Cards with older chipsets can't process more than 400-500MiB/s with RAID. You definitely want to check for that.

In the end, only you can answer this question by benchmarking. Check how fast either configuration is running and use it.

I'd advise against RAID5/6, especially when it's a performance critical array.

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Thanks, yes I plan on ordering and benchmarking. There isn't that much info online about people trying this so I wanted to make sure there weren't any big gotchas before proceeding with the usual validation steps. – root Nov 26 '12 at 1:02
@root: Then make sure you've got BBU in your controller, most higher end devices won't even allow to set up RAID 5 if you don't have BBU, but I haven't worked with Dell hardware. Besides that and possible bottleneck on the controller chipset I don't think there are any gotchas. – Hubert Kario Nov 26 '12 at 4:59

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