From: Christoph Hellwig <hch@infradead.org>
To: Victor Stinner <victor.stinner@haypocalc.com>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: Issues with KVM
Date: Mon, 25 Jul 2011 12:22:57 -0400 [thread overview]
Message-ID: <20110725162257.GA31516@infradead.org> (raw)
In-Reply-To: <4E2D7801.80207@haypocalc.com>
On Mon, Jul 25, 2011 at 04:04:49PM +0200, Victor Stinner wrote:
> According to agraf__ on IRC (#kvm on FreeNode), the cache mode has
> the following effect:
>
> - cache=writethrough calls fsync() after every write()
It uses O_DSYNC, which an be appromiately described as an fdatasync
after every write. Not that it makes a different for most workloads on
btrfs.
> - cache=none uses O_DIRECT
> - cache=writeback calls fsync() when the guest issues a barrier()
> (don't use O_DIRECT)
Issue a cache flush. Barriers were an Linux-internal concept that
is now gone. And there never was a barriere() call.
> FreeBSD installation in VirtualBox is as fast (or maybe a little bit
> slower) than the installation in kvm using cache=unsafe. I suppose
> that VirtualBox uses something like cache=unsafe or cache=writeback.
What filesystem do you use in FreeBSD? FFS traditionally never issued
cache flushes, so cache=writeback is equal to cache=unsafe for it. ZFS
can issue cache flushes on Solaris, but I'm not sure if this was ported
correctly to FreeBSD.
next prev parent reply other threads:[~2011-07-25 16:22 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-22 19:44 Issues with KVM Victor Stinner
2011-07-22 19:59 ` C Anthony Risinger
2011-07-22 20:00 ` C Anthony Risinger
2011-07-23 9:20 ` Victor Stinner
2011-07-25 14:04 ` Victor Stinner
2011-07-25 16:22 ` Christoph Hellwig [this message]
2011-07-22 20:44 ` Josef Bacik
2011-07-22 21:03 ` Morten P.D. Stevens
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