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Re: [Qemu-devel] [PATCH 1/5] block: add discard support
From: |
Paul Brook |
Subject: |
Re: [Qemu-devel] [PATCH 1/5] block: add discard support |
Date: |
Mon, 13 Dec 2010 16:17:42 +0000 |
User-agent: |
KMail/1.13.5 (Linux/2.6.36-trunk-amd64; KDE/4.4.5; x86_64; ; ) |
> On Sat, Dec 11, 2010 at 12:50:20PM +0000, Paul Brook wrote:
> > > It's guest visible state, so it must not change due to migrations. For
> > > the current implementation all values for it work anyway - if it's
> > > smaller than the block size we'll zero out the remainder of the block.
> >
> > That sounds wrong. Surely we should leave partial blocks untouched.
>
> While zeroing them is not required for qemu, the general semantics of
> the XFS ioctl require it. It punches a hole, which means it's makes the
> new area equivalent to a hole create by truncating a file to a larger
> size and then only writing at the larger offset. The semantics for a
> hole in all Unix filesystems is that we read back zeroes from them.
> If we write into a sparse file at a not block aligned offset the
> zeroing of the partial block also happens.
Ah, so it was just inconsistent use of the term "block". When the erase
region includes part of a block, we zero that part of the block and leave the
rest of the block untouched.
Paul
[Qemu-devel] [PATCH 2/5] scsi-disk: support WRITE SAME (16) with unmap bit, Christoph Hellwig, 2010/12/01
[Qemu-devel] [PATCH 3/5] make dma_bdrv_io available to drivers, Christoph Hellwig, 2010/12/01
[Qemu-devel] [PATCH 4/5] ide: add TRIM support, Christoph Hellwig, 2010/12/01
[Qemu-devel] [PATCH 5/5] raw-posix: add discard support, Christoph Hellwig, 2010/12/01