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

[Sylvain Munaut <s.munaut@xxxxxxxxxxxxxxxxxxxx>]

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 ?


Cheers,

    Sylvain

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