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Re: [Xen-devel] IO speed limited by size of IO request (for RBD driver)

  • To: xen-devel@xxxxxxxxxxxxx
  • From: Steven Haigh <netwiz@xxxxxxxxx>
  • Date: Tue, 23 Apr 2013 23:41:57 +1000
  • Delivery-date: Tue, 23 Apr 2013 13:43:02 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

On 23/04/2013 11:33 PM, Sylvain Munaut wrote:

I was observing a pretty severe performance impact when using Xen VM
with RBD (Ceph) backed storage, especially when doing large sequential

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@xxxxxxxxx
Web: https://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897
Fax: (03) 8338 0299

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