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Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_tr


From: Jeff Cody
Subject: Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_truncate if we are shrinking
Date: Wed, 6 Mar 2013 13:48:00 -0500
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Mar 06, 2013 at 07:31:51PM +0100, Paolo Bonzini wrote:
> Il 06/03/2013 19:14, Jeff Cody ha scritto:
> > QCOW breaks with it using a normal raw posix file as a device.  As a
> > test: qemu-img create -f qcow test.qcow 5G.  Now run qemu with that
> > drive mounted, and try to partition and format it.  QEMU now asserts.
> > 
> > The nicety of being able to using truncate during a write call,
> > especially for VHDX (which can have relatively large block/cluster
> > sizes), so to grow the file sparsely in a dynamically allocated file.
> 
> Perhaps we need two APIs, "truncate" and "revalidate".
> 
> Truncate should be a no-op if (!bs->growable).
> 
> Revalidate could be called by the block_resize monitor command with no
> size specified.
> 
> Paolo

I think that is a good solution.  Is it better to have "truncate" and
"revalidate", or "truncate" and "grow", with grow being a subset of
truncate, with fewer restrictions?  There may still be operations
where it is OK to grow a file, but not OK to shrink it.



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