From: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>,
qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH v3 2/6] block: add .bdrv_co_write_zeroes() interface
Date: Thu, 22 Dec 2011 07:54:43 +0000 [thread overview]
Message-ID: <20111222075443.GB8758@stefanha-thinkpad.localdomain> (raw)
In-Reply-To: <20111221165032.GA27022@lst.de>
On Wed, Dec 21, 2011 at 05:50:32PM +0100, Christoph Hellwig wrote:
> On Wed, Dec 21, 2011 at 04:00:36PM +0000, Stefan Hajnoczi wrote:
> > The ability to zero regions of an image file is a useful primitive for
> > higher-level features such as image streaming or zero write detection.
> >
> > Image formats may support an optimized metadata representation instead
> > of writing zeroes into the image file. This allows zero writes to be
> > potentially faster than regular write operations and also preserve
> > sparseness of the image file.
> >
> > The .bdrv_co_write_zeroes() interface should be implemented by block
> > drivers that wish to provide efficient zeroing.
> >
> > Note that this operation is different from the discard operation, which
> > may leave the contents of the region indeterminate. That means
> > discarded blocks are not guaranteed to contain zeroes and may contain
> > junk data instead.
>
> Most real life discard operations zero the data, and both the ATA and SCSI
> spec allow the device to set a bit which gurantees this behaviour. I think
> we also should make these one interface, and if the caller needs it to
> actually zero out the discarded blocks it should check if the discard
> implementation guarantees that.
Okay, I see how that could work but still need to look into the details
of how to combine the two and check the zero/indeterminate bit coming
from ATA/SCSI.
Stefan
next prev parent reply other threads:[~2011-12-22 9:39 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-21 16:00 [Qemu-devel] [PATCH v3 0/6] block: zero writes Stefan Hajnoczi
2011-12-21 16:00 ` [Qemu-devel] [PATCH v3 1/6] cutils: extract buffer_is_zero() from qemu-img.c Stefan Hajnoczi
2011-12-21 16:43 ` Eric Blake
2011-12-22 7:47 ` Stefan Hajnoczi
2011-12-21 16:00 ` [Qemu-devel] [PATCH v3 2/6] block: add .bdrv_co_write_zeroes() interface Stefan Hajnoczi
2011-12-21 16:50 ` Christoph Hellwig
2011-12-22 7:54 ` Stefan Hajnoczi [this message]
2011-12-21 16:00 ` [Qemu-devel] [PATCH v3 3/6] block: perform zero-detection during copy-on-read Stefan Hajnoczi
2011-12-21 16:00 ` [Qemu-devel] [PATCH v3 4/6] qed: replace is_write with flags field Stefan Hajnoczi
2011-12-21 16:00 ` [Qemu-devel] [PATCH v3 5/6] qed: add .bdrv_co_write_zeroes() support Stefan Hajnoczi
2011-12-21 16:00 ` [Qemu-devel] [PATCH v3 6/6] qemu-io: add write -z option for bdrv_co_write_zeroes Stefan Hajnoczi
2011-12-21 16:03 ` [Qemu-devel] [PATCH v3 0/6] block: zero writes Stefan Hajnoczi
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