[Xen-users] XEN 3.1 on 2.6.18-164.el5xen kernel - Slow IO & Pauses

["Robert Taylor" <rob@xxxxxxxx>]

I've seen tons of posts about slow IO under Xen and others without slow IO
under xen.   Here's what I'm experiencing.   I've got a XEN system on an 2
dual core AMD Opteron 275 processors.

I'm running DRBD for disk, but have also tested without DRBD.
Throughput in Dom-0 seems to be 50-80MB/sec for a file of any size.
Throughput in DomU is 20-30MB/sec for a file under 2GB, and 1-2MB/sec for
a larger file.

I've seen many other posts which are similar.  I've tried diagnosing and
I'm seeing the the IO seems to be throttled somewhere between the DomU and
the Dom0.   (Since they're LVM partitions, I'm assuming Dom0 handles the
LVM, because there is LVM xen module.)

The strange thing I'm finding is that under the DomU, when the IO is being
blocked, its blocking the entire kernel.   In other words, even non-disk
bound processes are pausing.   For instance if you ssh to the box, even
the network IO for SSH will pause for 10-20 seconds before it responds.  
Its very strange.   I was on 3.0 and didn't seem to see it as bad as when
I moved to 3.1.

xm info:

release                : 2.6.18-164.6.1.el5xen
version                : #1 SMP Tue Nov 3 17:53:47 EST 2009
machine                : i686
nr_cpus                : 4
nr_nodes               : 1
sockets_per_node       : 2
cores_per_socket       : 2
threads_per_core       : 1
cpu_mhz                : 2210
hw_caps                :
178bfbff:e3d3fbff:00000000:00000010:00000001:00000000:00000003
total_memory           : 7422
free_memory            : 0
node_to_cpu            : node0:0-3
xen_major              : 3
xen_minor              : 1
xen_extra              : .2-164.6.1.el5
xen_caps               : xen-3.0-x86_32p
xen_pagesize           : 4096
platform_params        : virt_start=0xf5800000
xen_changeset          : unavailable
cc_compiler            : gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)
cc_compile_by          : mockbuild
cc_compile_domain      : centos.org
cc_compile_date        : Tue Nov  3 16:10:34 EST 2009
xend_config_format     : 2


Any help would be appreciated.  This is under CentOS 5.4, 8GB memory, 1TB
SATA nv-sata controller.   Tyan K8SRE S2891 motherboard.   NVidia nForce
4Port SATAlink controller.



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