Messages in this thread | | | From | Neil Brown <> | Date | Tue, 22 Nov 2005 08:56:45 +1100 | Subject | Re: Poor Software RAID-0 performance with 2.6.14.2 |
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On Monday November 21, lroland@gmail.com wrote: > I have created a stripe across two 500Gb disks located on separate IDE > channels using: > > mdadm -Cv /dev/md0 -c32 -n2 -l0 /dev/hdb /dev/hdd > > the performance is awful on both kernel 2.6.12.5 and 2.6.14.2 (even > with hdparm and blockdev tuning), both bonnie++ and hdparm (included > below) shows a single disk operating faster than the stripe: > > ---- > dkstorage01:~# hdparm -t /dev/md0 > /dev/md0: > Timing buffered disk reads: 182 MB in 3.01 seconds = 60.47 MB/sec > > dkstorage02:~# hdparm -t /dev/hdc1 > /dev/hdc1: > Timing buffered disk reads: 184 MB in 3.02 seconds = 60.93 MB/sec > ----
Could you try hdparm tests on the two drives in parallel? hdparm -t /dev/hdb & hdparm -t /dev/hdd
It could be that the controller doesn't handle parallel traffic very well.
> > I am aware of cpu overhead with software raid but such a degradation > should not be the case with raid 0, especially not when the OS is > located on a separate SCSI disk - the IDE disks should just be ready > to work.
raid0 has essentially 0 cpu overhead. It would be maybe a couple of hundred instructions which would be lost in the noise. It just figures out which drive each request should go to, and directs it there.
> > There have been some earlier reporting on this problem but they all > seam to end more and less inconclusive (here is one > http://kerneltrap.org/node/4745). Some people favors switching to > dmraid with device mapper, is this the de facto standard today ? >
The kerneltrap reference is about raid5. raid5 is implemented very differently to raid0.
It might be worth experimenting with different read-ahead values using the 'blockdev' command. Alternately use a larger chunk size.
I don't think there is a de facto standard. Many people use md. Many use dm.
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