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Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE t
Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming
Sat, 24 Mar 2012 16:40:34 +0100
On Fri, Mar 09, 2012 at 02:36:50PM -0600, Richard Laager wrote:
> I'm not sure if fallocate() and/or BLKDISCARD always guarantee that the
> discard has made it to stable storage. If they don't, does O_DIRECT or
> O_DSYNC on open() cause them to make any such guarantee? If not, should
> you be calling fdatasync() or fsync() when the user has specified
> cache=direct or cache=writethrough?
> Note that the Illumos implementation (see below) has a flag to ask for
> either behavior.
For fallocate the current Linux behaviour is that you need an fdatasync
or the O_DSYNC flag to guarantee that it makes it's way to disk. For
XFS the history implementation of both XFS_IOC_FREESP and hole punching
was that it always made it to disk, and for all other filesystems that
historic behaviour was that O_DSYNC was ignored, and depending on the
fs a fdatasync might or might not be able to give your a guarantee either.
For BLKDISCARD you'd strictly speaking need to issue a cache flush via
fdatasync, too, but I'm not sure if any actual devices require that.
> On Fri, 2012-03-09 at 00:35 -0800, Chris Wedgwood wrote:
> > Simplest still compare the blocks allocated by the file
> > to it's length (ie. stat.st_blocks != stat.st_size>>9).
> I thought of this as well. It covers "99%" of the cases, but there's one
> case where it breaks down. Imagine I have a sparse file backing my
> virtual disk. In the guest, I fill the virtual disk 100%. Then, I
> restart QEMU. Now it thinks that sparse file is non-sparse and stops
> issuing hole punches.
Note that due to speculative preallocation in the filesystem stat.st_blocks
can easily be bigger than stat.st_size>>9, so the above comparism should
at least be a < instead of !=.
Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming, Stefan Hajnoczi, 2012/03/09
Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming, Richard Laager, 2012/03/09
[Qemu-devel] [RFC PATCH 17/17] raw: implement is_allocated, Paolo Bonzini, 2012/03/08
[Qemu-devel] [RFC PATCH 13/17] block: fallback from discard to writes, Paolo Bonzini, 2012/03/08
- Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming, (continued)