On Wed, Apr 25, 2012 at 01:47:23PM +0200, Lars Täuber spake thusly: > Tracy Reed <tr...@ultraviolet.org> schrieb: > > Reasons why I am currently migrating away from AoE to iSCSI (*sigh*): > > > > 1. Disk alignment between Xen VMs and the target. > > What do you mean by that?
I mean that when I configure a Xen VM to use an AoE block device I always had mis-aligned writes. The Xen dom0 has the block device in /dev/etherd and I put the block device in the Xen VM config file and make it /dev/xvda inside the VM. If I access the device from dom0 everything is fine. Very fast writes, no misalignment. But accessing the block device from within the VM causes the problem. This makes no sense to me and I don't see anything that could cause alignment to change but it clearly did somehow. This got to be very noticeable performance-wise and when doing a pure-write benchmark while running iostat on the target I could see lots of reads happening to backfill partial pages due to the misaligned write. -- Tracy Reed
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