Carlo Wood wrote:
The following can be observed:
1) There is hardly any difference between the two schedulers (noop
is a little faster for the bonny test).
2) An NCQ depth of 1 is WAY faster on RAID5 (bonnie; around 125 MB/s),
the NCQ depth of 2 is by far the slowest for the RAID5 (bonnie;
around 40 MB/s). NCQ depths of 3 and higher show no difference,
but are also slow (bonnie; around 75 MB/s).
3) There is no significant influence of the NCQ depth for non-RAID,
either the /dev/sda (hdparm -t) or /dev/sdd disk (hdparm -t and
bonnie).
4) With a NCQ depth > 1, the hdparm -t measurement of /dev/md7 is
VERY unstable. Sometimes it gives the maximum (around 150 MB/s),
and sometimes as low as 30 MB/s, seemingly independent of the
NCQ depth. Note that those measurement were done on an otherwise
unloaded machine in single user mode; and the measurements were
all done one after an other. The strong fluctuation of the hdparm
results for the RAID device (while the underlaying devices do not
show this behaviour) are unexplainable.
From the above I conclude that something must be wrong with the
software RAID implementation - and not just with the harddisks, imho.
At least, that's what it looks like to me. I am not an expert though ;)
I'm late tuning in here, but:
(1) hdparm issues only a single read at a time, so NCQ won't help it.
(2) WD Raptor drives automatically turn off "read-ahead" when using NCQ,
which totally kills any throughput measurements. They do this to speed
up random access seeks; dunno if it pays off or not. Under Windows,
the disk drivers don't use NCQ when performing large I/O operations,
which avoids the performance loss.
(3) Other drives from other brands may have similar issues,
but I have not run into it on them yet.
Cheers
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