Re: [Qemu-devel] [PATCH v1 04/13] qcow2: preallocation at image expand

[Anton Nefedov]

On 05/22/2017 10:29 PM, Eric Blake wrote:
> On 05/19/2017 04:34 AM, Anton Nefedov wrote:
> > From: "Denis V. Lunev" <address@hidden>
> >
> > This patch adds image preallocation at expand to provide better locality
> > of QCOW2 image file and optimize this procedure for some distributed
> > storages where this procedure is slow.
> >
> > Image expand requests have to be suspended until the allocation is
> > performed which is done via special QCowL2Meta.
> > This meta is invisible to handle_dependencies() code.
> > This is the main reason for also calling preallocation before metadata
> > write: it might intersect with preallocation triggered by another IO,
> > and has to yield
>
> How does this interact with Max's work on preallocated truncate?
> https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg00267.html

Seems like Max's patchset makes it possible to manually (with qemu-img)
allocate a continuous guest address space in advance?

Preallocation in this series is a little bit different:
- it's in-flight, and it fallocates (using drv->write_zeroes) space
ahead in the underlying image as qcow2 writes beyond EOF.
- it doesn't link L2. So it leaves the image 'sparse' on qcow2 level.

If you ask me, these 2 patchsets are not contradictory, and probably
won't even merge-conflict :)

/Anton