SSDs vs elevators (was Re: cap on writeback?)

[arlie@worldash.org (Arlie Stephens)]

On Mar 25 2013, Valdis.Kletnieks at vt.edu wrote:
> 
> Just for the record, most of my high-performance stuff runs best with
> the noop scheduler - when you're striping I/O across several hundred disks,
> the last thing you want is some some single-minded disk scheduler re-arranging
> the I/Os and creating latency issues for your striping.
> 
> Might want to think about why there's lots of man-hours spent doing
> new filesystems and stuff like zcache and kernel shared memory, but the
> only IO schedulers in tree are noop, deadline, and cfq :)

Interesting. I'd have expected, naively, that when dealing with SSDs -
and their delightful habits of rearranging data under the covers,
suddenly introducing delays, as well as their desire to have reads
grouped away from writes (RWRWR == slower than RRRWW if I understand
correctly) there'd be work to be done at the IO scheduler layer, not
just at the file systems layer, to get the best performance out of SSDs.

For the record, my own attempts to get performance out of SSDs have
been productive of nothing but pain. I was, however, trying to do this
_without_ kernel mods, and on FreeBSD rather than linux. 

--
Arlie