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We have this issue as well, but mostly on NFS based repositories. It's still under investigation with Oracle.
What type of storage did 2.2 have, and has that changed in your 3.1 environment? Is your 3.1 environment completely up to the latest patch level (higher than release 309)?
There was a pretty big dm-nfs issue with early 3.1 release that was resolved that caused huge NFS slowdowns.
We're patched up to the current patch level. We're using the exact same storage type as 2.2 (iSCSI), just a different LUN. The 2.2 multipath.conf file settings were used for the 3.1 multipath.conf.
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