Hi Stan,

(Thanks everyone else who's responded so far, too -- I'm paying
attention with interest)

On Thu, Oct 10, 2013 at 04:15:08AM -0500, Stan Hoeppner wrote:

On 10/9/2013 7:31 AM, Andy Smith wrote:

quoted

Are there any gotchas to be aware of? I haven't much experience with
SSDs.

Yes, there is one major gotcha WRT md/RAID and SSDs, which to this point
nobody has mentioned in this thread, possibly because it pertains to
writes, not reads.  Note my question posed to you up above.  Since I've
answered this question in detail at least a dozen times on this mailing
list, I'll simply refer you to one of my recent archived posts for the
details:

http://permalink.gmane.org/gmane.linux.raid/43984

When I first read that link I thought perhaps you were referring to
write performance dropping off a cliff due to SSD garbage caching
routines that kicked in, but then I read the rest of the thread and
I think maybe you were hinting at the single write thread issue you
talk about more in:

    http://www.spinics.net/lists/raid/msg44211.html

Is that the case?

To be clear, the need for careful directory/file layout to achieve
parallel throughput pertains only to the linear concatenation storage
architecture described above.  If one is using XFS atop a striped array
then throughput, either sequential or parallel, is -not- limited by
file/dir placement across the AGs, as all AGs are striped across the disks.

So, in summary do you recommend the stacked RAID-0 on top of RAID-1
pairs instead of a RAID-10, where write performance may otherwise be
bottlenecked by md's single write thread?

Write ops are a fraction of the random reads and using RAID with a
battery-backed write cache solved that problem, but it does need to
scale linearly with whatever improvement we can get for the read
ops, so I would think it will still be something worth thinking
about, so thanks for pointing that out.

Thanks,
Andy