Messages in this thread | | | Date | Sun, 24 Jun 2007 19:46:58 -0400 | From | Mark Lord <> | Subject | Re: SATA RAID5 speed drop of 100 MB/s |
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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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