slow read IO in domU

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slow read IO in domU

desire-4
Hi,  I think I must've done something incorrectly, but I can't quite figure out what it is.

To compare disk I/O performance in dom0 and domU, I ran the following two dd lines and observed vmstat 1 results (for blocks read/written per second), dd's reported speed, etc.  both dom0 and domU are allocated 512MB of RAM, so reading the 1GB file should not benefit from caching.

dd bs=1000000 count=1000 if=/dev/zero of=junk
(sync, wait, etc)
dd bs=1000000 count=1000 of=/dev/zero if=junk

In dom0, I achieve 45MB/s write speed and 63MB/s read speed reasonably consistently.  I guess this should be reasonable speeds for a partition on LVM on linux software raid 0 on a pair of SATA drives near the front of the disk.  This speed was observed using dd output as well as dom0 vmstat.

In domU, I achieve also about 45MB/s write speed but only about 13MB/s read speed consistently, while writing to a "sda3" residing on a dedicated LVM volume on software raid 0 on the same part of disks.  This speed was observed in vmstat on both dom0 and domU.

This is on xen 2.0.6 with linux 2.6.9 dom0 and domU kernels.  Shouldn't dom0 and domU achieve similar read & write speeds? Thanks for any pointers as to where I've gone wrong...


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