From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sylvain Munaut <s.munaut@whatever-company.com>
Subject: IO speed limited by size of IO request (for RBD driver)
Date: Tue, 23 Apr 2013 15:33:28 +0200
Message-ID: <CAF6-1L5fePSH1eddGSa5e9ZChuNp3Gdph-wrfjJo75Ts=9Ra4Q@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <xen-devel-bounces@lists.xen.org>
List-Unsubscribe: <http://lists.xen.org/cgi-bin/mailman/options/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xen.org>
List-Help: <mailto:xen-devel-request@lists.xen.org?subject=help>
List-Subscribe: <http://lists.xen.org/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=subscribe>
Sender: xen-devel-bounces@lists.xen.org
Errors-To: xen-devel-bounces@lists.xen.org
To: xen-devel@lists.xen.org
List-Id: xen-devel@lists.xenproject.org

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