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

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

On 23/04/2013 11:33 PM, Sylvain Munaut wrote:
> Hi,
>
>
> I was observing a pretty severe performance impact when using Xen VM
> with RBD (Ceph) backed storage, especially when doing large sequential
> access.
>
> And I think I finally found a major cause for it: even large user
> space requests seem to be split into small requests of 11 * 4096
> bytes. ( 44k )
>
> This is caused by :
>
>          blk_queue_max_segments(rq, BLKIF_MAX_SEGMENTS_PER_REQUEST);
>          blk_queue_max_segment_size(rq, PAGE_SIZE);
>
> What's the impact of modifying those ?
> I've seen some justification for the BLKIF_MAX_SEGMENTS_PER_REQUEST
> limit, but why limit segment size to page_size ?

I'm seeing the same as you - see the thread in the archives over the 
last few weeks - subject "RE: Xen disk write slowness in kernel 3.8.x"

I get ~50MB/sec max write speeds due to probably the same problem.

-- 
Steven Haigh

Email: netwiz@crc.id.au
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