Very inconsistent (read) speed on UFS2
daniel at digsys.bg
Wed Aug 31 08:48:42 UTC 2011
On 31.08.11 11:36, Lev Serebryakov wrote:
> device r/s w/s kr/s kw/s wait svc_t %b
> ada1 340.9 292.9 43138.8 146.5 0 1.2 42
> ada2 340.9 293.9 43138.8 147.0 0 1.9 63
> ada3 340.9 292.9 43044.7 146.5 0 1.5 57
> ada4 341.9 292.9 43232.9 146.5 0 1.3 42
> ada5 341.9 292.0 43138.8 146.0 2 1.3 40
Very interesting, this writes. You need to find out what is causing these.
Just some random thoughts:
This flapping may have something to do with the drives' internal caches.
What are the drives?
SATA drives, unlike SAS have simplex communication with the host, that
is, the drive cannot simultaneously read and write data and commands
(from/to host). There might be some, perhaps locking contention in
there? It is not contention for bandwidth obviously.
Most consumer drives have rather low IOPS performance. This is
especially pronounced when there are both reads and writes. Your IOPS
rate here is relatively high for such drive, although the busy
percentage is low -- but then, it may not be accurate.
In any case, you cannot measure read performance as long as it
intermixes with writes, especially as you noted that your RAID5 code has
some non-obvious write characteristics/optimizations.
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