On 29/09/17 18:22, Joe Landman wrote:
On 09/29/2017 11:53 AM, Dag Nygren wrote:
Hi all!
Would like to tap some experience out of all here
with the question:
Any good hints and advice when setting up a RAID5 SSD with
3 disks to start with?
You would need to worry about write amplification due to R5. So if you
do this, use SSDs with higher DWPD (drive writes per day). Aim for
3DWPD if you can, so you don't burn out the SSDs early. Don't do this
with consumer grade SSDs (anything 0.5 DWPD or less). They do burn out
(sometimes much) faster. The little extra money spent on the enterprise
SATA (or SAS) with higher DWPD is worth it.
Precondition the SSDs. If you don't know how, I wrote a nice little
util here: https://github.com/joelandman/disk_test_setup that helps you
do it ... uses fio to drive 128k seqeuential writes to fill drives.
Drive life appears well correlated with preconditioning and write loads.
Use a chunk size of 128k or so (larger better). You want the chunk size
at the same size as the erase block size. Reduces write amplification.
You still have to worry about the whole RMW cycle for RAID5. This
means, for small IO (below chunk/erase block size), you have to
read-modify-write at least 2 blocks back for every block written. If
your writes are small (4k -> 32k) you'll want to invest in even higher
quality (e.g. more DWPD).
If you can get enough drives, I'd actually recommend a RAID10. Much
lower write amplification -> longer lifetime.
And also much faster. The key point for speed in SSD's is low latency -
and RAID5 can add a lot of latency to writes.
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