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Re: [Qemu-devel] [PATCH v3 2/6] block: add .bdrv_co_write_zeroes() inter
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v3 2/6] block: add .bdrv_co_write_zeroes() interface |
Date: |
Thu, 22 Dec 2011 07:54:43 +0000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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
- [Qemu-devel] [PATCH v3 0/6] block: zero writes, Stefan Hajnoczi, 2011/12/21
- [Qemu-devel] [PATCH v3 1/6] cutils: extract buffer_is_zero() from qemu-img.c, Stefan Hajnoczi, 2011/12/21
- [Qemu-devel] [PATCH v3 3/6] block: perform zero-detection during copy-on-read, Stefan Hajnoczi, 2011/12/21
- [Qemu-devel] [PATCH v3 4/6] qed: replace is_write with flags field, Stefan Hajnoczi, 2011/12/21
- [Qemu-devel] [PATCH v3 2/6] block: add .bdrv_co_write_zeroes() interface, Stefan Hajnoczi, 2011/12/21
- [Qemu-devel] [PATCH v3 5/6] qed: add .bdrv_co_write_zeroes() support, Stefan Hajnoczi, 2011/12/21
- [Qemu-devel] [PATCH v3 6/6] qemu-io: add write -z option for bdrv_co_write_zeroes, Stefan Hajnoczi, 2011/12/21
- Re: [Qemu-devel] [PATCH v3 0/6] block: zero writes, Stefan Hajnoczi, 2011/12/21