[Xen-users] XCP and tapdisk

[Eduardo Bragatto <eduardo@xxxxxxxxxxxx>]

Hello,

I recently installed XCP on a Dell R710, using an MD3200i storage with 2 x 1Gbps connections from the host to the storage.

I got multipath working fine and using both links in round-robin with no problem. If I use a small LUN (about 100GB) and create a filesystem on it, I can write up to 2Gbps as expected (a bit less due to overhead, of course, but write speeds stay around 220MB/s to 224MB/s).

Writing speed measured using: dd if=/dev/zero of=file.img bs=1M count=10K

However, if I use the same LUN to create an SR on XCP and install a VM on it, the same "dd" command as above, returns writing speeds about 130MB/s to 140MB/s when executed from the VM. I checked and it does use both paths available (2 x 1Gbps). I also noticed a process on dom0 called "tapdisk" eating up to 50% CPU during the time the VM was writing to the disk.

So, it seems "tapdisk" my bottleneck here, however I couldn't find much documentation on it. All I've found is that it seems to exsite alternatives to it, like tapdisk2, is that right?

Where can I find further information on tapdisk and how to improve I/O performance on a XCP hypervisor?

PS: I'm coming from CentOS 5 with Xen 3, and by using "disk = [ phy:/dev/mapp/mpath..." I got the same performance speed as I mentioned in the first scenario, but inside the VM. Is there a way to avoid tapdisk entirely by using something like "phy" under XAPI / XCP ?

Thanks,
Eduardo Bragatto

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