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Date: Mon, 13 Dec 2010 16:43:57 +0100
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] [PATCH 1/5] block: add discard support
Message-ID: <20101213154357.GA5730@lst.de>
References: <20101125135657.GA2814@lst.de> <4CF78D1D.3060107@redhat.com>
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	<201012111250.20905.paul@codesourcery.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Christoph Hellwig <hch@lst.de>

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.