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ZFS Mirroring/Stripping

4

402blownstroker

[H]ard|DCer of the Month - Nov. 2012
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I have a question of what is the most optimal way to configure drives. I have a RAID controller that is SMART enabled, so arrays status can be reported back to the OS. For performance wise what would be a better setup:

1) Mirror drives with the RAID controller and then stripe them with ZFS.
2) Stripe them with the RAID controller and then mirror them with ZFS.

Size wise I think the pool will work out to be the same capacity. I will also have a couple of SATA-III SSDs as cache. With the SSD caches, does this question become moot?
 
If you mirror the drives with the RAID controller and stripe them in ZFS, you lose the error correction capabilities of ZFS.
The best way to do this is not to use a RAID controller at all and do mirroring and striping entirely in ZFS.

"A couple" (more than a single) of SSDs for cache may not help depending on your workload. With ZFS, the SSDs are an extension of the RAM cache and the contents are lost on reboots.
More L2ARC space will also consume more RAM. Most of the time the money is better invested in more RAM.
 
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Note though that illumos has recently added (IIRC) persistent l2arc devices. I believe this has also been merged into ongoing ZoL work.
 
omniscence said:
If you mirror the drives with the RAID controller and stripe them in ZFS, you lose the error correction capabilities of ZFS.
The best way to do this is not to use a RAID controller at all and do mirroring and striping entirely in ZFS.
Click to expand...

OK, I can understand the aguement for letting ZFS handling the mirror, but how would software stripping( aka ZFS ) have better performance than hardware stripping?
 
Not to nitpick (okay, I am), it's striping, not stripping. Secondly, many (if not most) HW raid cards do not load balance when doing reads. ZFS does.
 
The CPU is much faster than the processor on RAID cards. And the RAID SoCs run on software, too. The only advantage I still see in dedicated RAID cards is a non-volatile write cache.

Putting ZFS mirrors on striped volumes will most likely cause bad alignment problems, unless you use a stripe width equal to the ZFS block size.
 
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