[dm-devel] An multipath performance issue on RHEL 5

[Konrad Rzeszutek]

On Tue, Jan 13, 2009 at 05:11:23PM +0800, dwu wrote:
> A customer has done a test of multipath on RHEL 5, and he found that the 
> speed is 30-40MB/sec, but it can reach 160MB/sec when using EMC powerpath 

Does it reach that when do a test on individual disks in the setup
with EMC powerpath?

>
> [root at clnode2 ~]# hdparm -t /dev/mapper/mpath0
>
> /dev/mapper/mpath0:
> Timing buffered disk reads: 118 MB in 3.01 seconds = 39.22 MB/sec
> [root at clnode2 ~]# hdparm -t /dev/mapper/mpath5
>
> /dev/mapper/mpath5:
> Timing buffered disk reads: 132 MB in 3.04 seconds = 43.38 MB/sec
> [root at clnode2 tmp]# hdparm -t /dev/sdm
>
> /dev/sdm:
> Timing buffered disk reads: 112 MB in 3.04 seconds = 36.89 MB/sec
> [root at clnode2 tmp]# hdparm -t /dev/sdaa
>
> /dev/sdaa:
> Timing buffered disk reads: 108 MB in 3.02 seconds = 35.81 MB/sec
> [root at clnode2 tmp]# hdparm -t /dev/sdf
>
> /dev/sdf:
> Timing buffered disk reads: read() failed: Input/output error
> [root at clnode2 tmp]# hdparm -t /dev/sdt
>
> /dev/sdt:
> Timing buffered disk reads: read() failed: Input/output error
>

Since you did the test on the underlaying SCSI subsystem (that
was the next thing to test) - which has no connection to multipath
it eliminates the multipath layer. When you do the hdparam test
on RHEL4 OS on those disks - are the numbers the same?

Is your RHEL4 rig the same exact machine with the same exact
fibre connection? You could have the RHEL5 using a 1GB connection
while the RHEL4 might be using 2GB?