Re: IO speed limited by size of IO request (for RBD driver)

[Steven Haigh <netwiz@crc.id.au>]

On 30/04/2013 8:07 PM, Felipe Franciosi wrote:
> I noticed you copied your results from "dd", but I didn't see any conclusions drawn from experiment.
>
> Did I understand it wrong or now you have comparable performance on dom0 and domU when using DIRECT?
>
> domU:
> # dd if=/dev/zero of=output.zero bs=1M count=2048 oflag=direct
> 2048+0 records in
> 2048+0 records out
> 2147483648 bytes (2.1 GB) copied, 25.4705 s, 84.3 MB/s
>
> dom0:
> # dd if=/dev/zero of=output.zero bs=1M count=2048 oflag=direct
> 2048+0 records in
> 2048+0 records out
> 2147483648 bytes (2.1 GB) copied, 24.8914 s, 86.3 MB/s
>
>
> I think that if the performance differs when NOT using DIRECT, the issue must be related to the way your guest is flushing the cache. This must be generating a workload that doesn't perform well on Xen's PV protocol.

Just wondering if there is any further input on this... While DIRECT 
writes are as good as can be expected, NON-DIRECT writes in certain 
cases (specifically with a mdadm raid in the Dom0) are affected by about 
a 50% loss in throughput...

The hard part is that this is the default mode of writing!

-- 
Steven Haigh

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