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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH 1/5] block: add discard support
Date: Mon, 13 Dec 2010 16:17:42 +0000
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To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org

> 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