Very interesting answer, thank you.
Partition alignment, and IO performance in general, is something I'm
trying to get
better grips on.
FWIW, we're on AoE 47 (stock Debian kernel), with AoE clients using
two AoE devices
as a backing device for a RAID1 device.
It works well enough, but performance isn't exactly stellar.
I have some questions that would be grateful to get the answer to.
* How will using iSCSI change the partition alignment problem ?
I'm guessing you have found the difference the only way that works; by trying.
It seems unclear why AoE would cause the unaligned writes, so maybe there isn't
a good answer to this question.
* How do you actually see from the output of iostat that you're writes
aren't aligned ?
I usually use "iostat -dxm 1" to get an overview, but I don't fully
understand all the output.
In a non-AoE context, I have a dm RAID6, where I see all the disks and
the md device
when using iostat.
I can there see the md device hitting 100% utilization, while the disk
don't go above ~60%.
Is that a symptom of unaligned writes ?
Thanks for your insightful answers.
On Thu, Apr 26, 2012 at 13:02, Tracy Reed <treed@...> wrote:
> On Wed, Apr 25, 2012 at 01:47:23PM +0200, Lars Täuber spake thusly:
>> Tracy Reed <treed@...> 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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