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SubjectRe: SATA RAID5 speed drop of 100 MB/s
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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