Re: Poor Software RAID-0 performance with 2.6.14.2

[Neil Brown]

On Monday November 21, lroland@xxxxxxxxx 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.  

NeilBrown
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