[Xen-users] Xen DomU poor I/O performance

[Michał Chybowski <michal.chybowski@xxxxxxxxxxxx>]

Hi

I'm working on SR for Xen using independent iSCSI targets for each VDI 
via IPoIB on Mellanox Technologies MT25208 [InfiniHost III Ex] (rev 20).
VDI is correctly attaching, detaching, etc. but it's I/O is terrible. 
VDI is attached to VM without use of blktap as with high I/O from VMs it 
can almost completely use up whole Dom0's CPU.
Using fio in DomU with --fsync=1 gives me exactly 0 IOPS.
If at the same time I'll run `ioping -D -i 0.01 filename` - fio shows I/O.
If I'll run `ping -f <ip of iSCSI target>`, I/O also "starts" to show up.
Also if I use dd with --oflag=dsync, I/O looks like above.

The same behaviour occurs when using classic 1Gbit/s Ethernet.

This does not happen if I log in the target in Dom0 and try the same 
tests with the same parameters there or if I'm using blktap (both on 
IPoIB and Ethernet).
Is there any method of finding source of this behaviour?

--
Regards
M


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