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Date: Tue, 8 Nov 2016 15:05:24 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] qemu-img create doesn't always replace the
 existing file
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org

[ Cc: qemu-block ]

Am 08.11.2016 um 11:58 hat Richard W.M. Jones geschrieben:
> When using 'qemu-img create', if the file being created already
> exists, then qemu-img tries to read it first.  This has some
> unexpected effects:
> 
> 
> $ rm test.qcow2 
> $ qemu-img create -f qcow2 -o compat=1.1,preallocation=off test.qcow2 1G
> Formatting 'test.qcow2', fmt=qcow2 size=1073741824 compat=1.1 encryption=off cluster_size=65536 preallocation=off lazy_refcounts=off refcount_bits=16
> $ du -sh test.qcow2 
> 196K test.qcow2
> 
> 
> $ rm test.qcow2 
> $ qemu-img create -f qcow2 -o compat=1.1,preallocation=falloc test.qcow2 1G
> Formatting 'test.qcow2', fmt=qcow2 size=1073741824 compat=1.1 encryption=off cluster_size=65536 preallocation=falloc lazy_refcounts=off refcount_bits=16
> $ qemu-img create -f qcow2 -o compat=1.1,preallocation=off test.qcow2 1G
> Formatting 'test.qcow2', fmt=qcow2 size=1073741824 compat=1.1 encryption=off cluster_size=65536 preallocation=off lazy_refcounts=off refcount_bits=16
> $ du -sh test.qcow2 
> 256K test.qcow2            # would expect this to be the same as above

For me it's actually even more:

$ du -h /tmp/test.qcow2 
448K    /tmp/test.qcow2

However...

$ ls -lh /tmp/test.qcow2 
-rw-r--r--. 1 kwolf kwolf 193K  8. Nov 15:00 /tmp/test.qcow2

So qemu-img can't be at fault, the file has the same size as always.

Are you using XFS? In my case I would have guessed that it's probably
some preallocation thing that XFS does internally. We've seen this
before that 'du' shows (sometimes by far) larger values than the file
size on XFS. That space is reclaimed later, though.

Kevin