From: Christoph Hellwig <hch@lst.de>
To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: xen-devel@lists.xensource.com, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH] qemu and qemu-xen: support empty write barriers in xen_disk
Date: Wed, 24 Nov 2010 17:58:35 +0100 [thread overview]
Message-ID: <20101124165835.GF31124@lst.de> (raw)
In-Reply-To: <alpine.DEB.2.00.1011241305080.2373@kaball-desktop>
I had the discussion with Jeremy in Boston before, but let's repeat it
here:
- is there actually any pre-existing xen backend that does properly
implement empty barries. Back then we couldn't find any.
- if this is a new concept to Xen please do not define an empty
barrier primitive, but a new flush cache primitive. That one
maps natively to the qemu I/O layer, and with recent Linux, NetBSD,
Windows, or Solaris guest will be a lot faster than a barrier
which drains the queue.
Note that what your patch implements actually is a rather inefficient
implementation of the latter. You do none of the queue draining which
the in-kernel blkback implementation does by submitting the old-style
barrier bio. While most filesystem do not care you introduce a quite
subtile chance of data corruption for reiserfs, or ext4 with
asynchronous journal commits on pre-2.6.37 kernels.
next prev parent reply other threads:[~2010-11-24 16:59 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-11-24 13:08 [Qemu-devel] [PATCH] qemu and qemu-xen: support empty write barriers in xen_disk Stefano Stabellini
2010-11-24 14:23 ` Gerd Hoffmann
2010-11-24 16:29 ` Kevin Wolf
2010-11-24 16:46 ` Stefano Stabellini
2010-12-14 18:43 ` [Xen-devel] " Ian Jackson
2010-11-24 16:58 ` Christoph Hellwig [this message]
2010-11-24 18:18 ` Jeremy Fitzhardinge
2010-11-24 18:44 ` Christoph Hellwig
2010-11-24 19:55 ` Stefano Stabellini
2010-11-26 11:03 ` Kevin Wolf
2010-11-26 13:29 ` Christoph Hellwig
2010-11-26 15:25 ` Stefano Stabellini
[not found] ` <m2n.s.1PLKM9-000VR4@chiark.greenend.org.uk>
2010-11-25 19:30 ` Ian Jackson
2010-11-25 19:46 ` Jeremy Fitzhardinge
2010-11-25 23:30 ` Christoph Hellwig
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