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We're having exactly the same problem as you appear to have experienced. Very slow write I/O in guest VMs (domU) but fast I/O on the host on the same iSCSI file system.
Write I/O inside the guest is between 3-20MB/sec (dd bs=2048k count=512), whereas on the host its 95MB/sec - which is hitting the practical limits of our GigEth iSCSI SAN.
I've checked inside the guests and can see no problems of write caches being disabled in dmesg. Curiously we hit this same wall on both a RHEL6.3 and Solaris10U10 guest.
I've tried both sparse and non-sparse files and the performance is the same. Read performance is fine, but this write bottleneck is a showstopper. Would appreciate any assistance you guys might have while I await a response from Oracle..
- Oracle VM 3.1.1 update 485
- Sun Fire X4150 Server
- Sun Storagetek 2510 iSCSI SAN