From: Paolo Bonzini <pbonzini@redhat.com>
To: Badari Pulavarty <pbadari@gmail.com>
Cc: linux-kernel@vger.kernel.org, rusty@rustcorp.com.au, kvm@vger.kernel.org
Subject: Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough
Date: Fri, 06 Jul 2012 08:47:39 +0200 [thread overview]
Message-ID: <4FF68A0B.8000402@redhat.com> (raw)
In-Reply-To: <1341513550.5508.7.camel@badari-desktop>
Il 05/07/2012 20:39, Badari Pulavarty ha scritto:
> On Tue, 2012-07-03 at 15:19 +0200, Paolo Bonzini wrote:
>> This patch adds support for the new VIRTIO_BLK_F_CONFIG_WCE feature,
>> which exposes the cache mode in the configuration space and lets the
>> driver modify it. The cache mode is exposed via sysfs.
>>
>> Even if the host does not support the new feature, the cache mode is
>> visible (thanks to the existing VIRTIO_BLK_F_WCE), but not modifiable.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>
> Hi Paolo,
>
> Curious - What is the host side change to support this ? QEMU would
> close and re-open the device/file with the corresponding flags
> (O_SYNC) ?
QEMU is not using O_SYNC anymore; instead, it manually issues a flush
after each write. We found this didn't penalize performance (in fact,
for qcow2 metadata writes the fdatasyncs will be more coarse and there
is a small improvement). So, when you toggle writethrough to writeback
QEMU simply has to stop forcing flushes, and vice versa if you go to
writethrough.
> And also, is there a way to expose cache=none (O_DIRECT) to the guest ?
Not yet. The main problem is that while we can invent something in
virtio-blk, I'm not sure if there is an equivalent in SCSI for example.
Paolo
prev parent reply other threads:[~2012-07-06 6:47 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-03 13:19 [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough Paolo Bonzini
2012-07-04 5:55 ` Rusty Russell
2012-07-04 6:24 ` Rusty Russell
2012-07-04 15:13 ` Michael S. Tsirkin
2012-07-04 15:42 ` Michael S. Tsirkin
2012-07-04 15:42 ` Michael S. Tsirkin
2012-07-04 15:54 ` Paolo Bonzini
2012-07-04 15:54 ` Paolo Bonzini
2012-07-04 16:02 ` Michael S. Tsirkin
2012-07-04 16:02 ` Michael S. Tsirkin
2012-07-04 16:08 ` Paolo Bonzini
2012-07-04 16:08 ` Paolo Bonzini
2012-07-04 21:30 ` Michael S. Tsirkin
2012-07-04 21:30 ` Michael S. Tsirkin
2012-07-05 6:45 ` Paolo Bonzini
2012-07-05 6:45 ` Paolo Bonzini
2012-07-04 16:26 ` Sasha Levin
2012-07-04 16:32 ` Paolo Bonzini
2012-07-04 21:11 ` Sasha Levin
2012-07-05 6:47 ` Paolo Bonzini
2012-07-05 7:02 ` Sasha Levin
2012-07-08 23:45 ` Rusty Russell
2012-07-25 1:02 ` Rusty Russell
2012-07-05 18:39 ` Badari Pulavarty
2012-07-06 6:47 ` Paolo Bonzini [this message]
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