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[Xen-users] iSCSI performance drop after Linux 2.6.38 ( tested 2.6.39 and 3.0.1 )

I know it is not directly Xen related but since the 3.0 Linux kernel
finally contains pretty much everything that you need to run your Dom0, 
I thought that I might share my observations with anybody who's using
iSCSI as storage backend for their Xen installation.

This is raw "dd" throughput for reading ~30GB from an iSCSI storage via
a dedicated 1GB Ethernet link.

2.6.32 : 102 MB/s
2.6.38 : 100 MB/s
2.6.39 : 44 MB/s
3.0.1 : 43 MB/s

Seems like between 2.6.38 and 2.6.39 the iSCSI performance got pretty
much thrown out the window ...

If I were into into conspiracy theories I'd suspect that the Core-iSCSI
guys are out to prove the superior performance of their initiator by
slowing down the open-iSCSI competitor. ;-)
(See http://linux-iscsi.org/wiki/Core-iSCSI#Performance )

I tested with Debian Squeeze on bare metal to remove Xen from the
equation and I tested without multipath to avoid those complications too.
Kernel 2.6.32 is the default Squeeze kernel, 2.6.38 and 2.6.39 were
from squeeze-backports and 3.0.1 was built from the vanilla kernel
sources using make-kpkg. The storage is a Dell / Equallogic PS4000.

Please drop me a line if you can confirm or refute those findings.

On the positive side I have to say that the 3.0.1 kernel is much more
stable in my Xen + iSCSI + multipath setup. (Though I havn't yet tested
it with the load that I have on the 2.6.32 machine.)


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