Re: [Qemu-devel] [PATCH 3/3] parallels: drop check that bdrv_truncate() is working

[Eric Blake <eblake@redhat.com>]

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On 08/04/2017 10:10 AM, Denis V. Lunev wrote:
> This would be actually strange and error prone. If truncate() nowadays
> will fail, there is something fatally wrong. Let's check for that during
> the actual work.
> 
> The only fallback case is when the file is not zero initialized. In this
> case we should switch to preallocation via fallocate().

I got confused by the commit message.  Here's my attempt an an
alternative, to see if I'm understanding the point of this patch:

The code was trying to truncate a file to its current length, as an
optimization to help decide whether an alternative prealloc mode was
useful.  But it forgot to check whether bdrv_getlength() succeeded, and
dealing with that failure just complicates what was supposed to be a
no-op probe for optimizing later operation.  We will still properly fail
later when an actual truncation attempt is made and the device can't
support it.  So when deciding how to set prealloc_mode while opening the
device, all we really need is to check just the one condition that
matters - knowing whether the device is zero initialized.

> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Markus Armbruster <armbru@redhat.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/parallels.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)

The code change makes sense at first glance, but I'm a bit reluctant to
give R-b, since the commit message threw me off and I'm not familiar
with the parallels code in the first place.

> 
> diff --git a/block/parallels.c b/block/parallels.c
> index 6794e53c0b..e1e06d23cc 100644
> --- a/block/parallels.c
> +++ b/block/parallels.c
> @@ -703,9 +703,7 @@ static int parallels_open(BlockDriverState *bs, QDict *options, int flags,
>          goto fail_options;
>      }
>  
> -    if (!(flags & BDRV_O_RESIZE) || !bdrv_has_zero_init(bs->file->bs) ||
> -            bdrv_truncate(bs->file, bdrv_getlength(bs->file->bs),
> -                          PREALLOC_MODE_OFF, NULL) != 0) {
> +    if (!bdrv_has_zero_init(bs->file->bs)) {
>          s->prealloc_mode = PRL_PREALLOC_MODE_FALLOCATE;
>      }
>  
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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