Re: [Qemu-devel] [PATCH] qemu-img: set nocow flag to new file

[Alex Bennée]

address@hidden writes:

> On Mon, Nov 18, 2013 at 12:54:59PM +0800, Chunyan Liu wrote:
>> 2013/11/15 Stefan Hajnoczi <address@hidden>
>> 
>> > On Thu, Nov 14, 2013 at 04:15:28PM +0800, Chunyan Liu wrote:
>> > > Set NOCOW flag to newly created images to solve performance issues on
>> > btrfs.
<snip>
>> > This should be optional and I'm not sure it should be the default.
>> >
>> > Rationale: If you're on btrfs you probably expect the copy-on-write and
>> > snapshot features of the file system.  We shouldn't silently disable
>> > that unless the user asks for it.
<snip>
>
> When the NOCOW attribute is set on a file, reflink copying (aka
> file-level snapshots) do not work:
>
> $ cp --reflink test.img test-snapshot.img
>
> This produces EINVAL.
>
> It is a regression if qemu-img create suddenly starts breaking this
> standard btrfs feature for existing users.
>
> Please make it a .bdrv_create() option which is off by default to avoid
> breaking existing users' workflows/scripts.  The result should be
> something like:
>
> $ qemu-img create test.img 8G # file has NOCOW cleared
> $ qemu-img create -o nocow=on test.img 8G # file has NOCOW set

I agree we shouldn't break existing work flows. I wonder if it would OK
for qemu-img to issue a warning (when not --quiet) when it detects
creation of an image on a partition where performance may not be as
expected due to COW behaviour.


Cheers,

--
Alex Bennée
QEMU/KVM Hacker for Linaro