Re: [PATCH v6 10/10] qcow2: Forward ZERO_WRITE flag for full preallocation

[Eric Blake <eblake@redhat.com>]

On 4/23/20 10:01 AM, Kevin Wolf wrote:
> The BDRV_REQ_ZERO_WRITE is currently implemented in a way that first the
> image is possibly preallocated and then the zero flag is added to all
> clusters. This means that a copy-on-write operation may be needed when
> writing to these clusters, despite having used preallocation, negating
> one of the major benefits of preallocation.
> 
> Instead, try to forward the BDRV_REQ_ZERO_WRITE to the protocol driver,
> and if the protocol driver can ensure that the new area reads as zeros,
> we can skip setting the zero flag in the qcow2 layer.

Hmm.  When we get block status, it is very easy to tell that something 
reads as zero when the qcow2 file has the zero bit set, but when the 
qcow2 file does not have the zero bit set, we have to then query the 
format layer whether it reads as zeros (which is expensive enough for 
some format layers that we no longer report things as reading as zero). 
I'm worried that optimizing this case could penalize later block status.

We already can tell the difference between a cluster that has the zero 
bit set but is not preallocated, vs. has the zero bit set and is 
preallocated.  Are we really forcing a copy-on-write to a cluster that 
is marked zero but preallocated?  Is the problem that we don't have a 
way to distinguish between 'reads as zeroes, allocated, but we don't 
know state of format layer' and 'reads as zeroes, allocated, and we know 
format layer reads as zeroes'?

> 
> Unfortunately, the same approach doesn't work for metadata
> preallocation, so we'll still set the zero flag there.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   block/qcow2.c              | 22 +++++++++++++++++++---
>   tests/qemu-iotests/274.out |  4 ++--
>   2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index ad621fe404..b28e588942 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -4170,9 +4170,25 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
>           /* Allocate the data area */
>           new_file_size = allocation_start +
>                           nb_new_data_clusters * s->cluster_size;
> -        /* Image file grows, so @exact does not matter */
> -        ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
> -                               errp);
> +        /*
> +         * Image file grows, so @exact does not matter.
> +         *
> +         * If we need to zero out the new area, try first whether the protocol
> +         * driver can already take care of this.
> +         */
> +        if (flags & BDRV_REQ_ZERO_WRITE) {
> +            ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc,
> +                                   BDRV_REQ_ZERO_WRITE, errp);
> +            if (ret >= 0) {
> +                flags &= ~BDRV_REQ_ZERO_WRITE;
> +            }

Hmm - just noticing things: how does this series interplay with the 
existing bdrv_has_zero_init_truncate?  Should this series automatically 
handle BDRV_REQ_ZERO_WRITE on a bdrv_co_truncate(PREALLOC_NONE) request 
for all drivers that report true, even if that driver does not advertise 
support for the BDRV_REQ_ZERO_WRITE flag?

> +        } else {
> +            ret = -1;
> +        }

Here, ret == -1 does not imply whether errp is set - but annoyingly, 
errp CAN be set if bdrv_co_truncate() failed.

> +        if (ret < 0) {
> +            ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
> +                                   errp);

And here, you are passing a possibly-set errp back to 
bdrv_co_truncate().  That is a bug that can abort.  You need to pass 
NULL to the first bdrv_co_truncate() call or else clear errp prior to 
trying a fallback to this second bdrv_co_truncate() call.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org