Re: [PATCH 10/17] block: Add new BDRV_ZERO_OPEN flag

[Eric Blake <eblake@redhat.com>]

On 1/31/20 11:44 AM, Eric Blake wrote:
> Knowing that a file reads as all zeroes when created is useful, but
> limited in scope to drivers that can create images.  However, there
> are also situations where pre-existing images can quickly be
> determined to read as all zeroes, even when the image was not just
> created by the same process.  The optimization used in qemu-img
> convert to avoid a pre-zeroing pass on the destination is just as
> useful in such a scenario.  As such, it is worth the block layer
> adding another bit to bdrv_known_zeroes().
> 
> Note that while BDRV_ZERO_CREATE cannot chase through backing layers
> (because it only applies at creation time, but the backing layer was
> not created at the same time as the active layer being created), it IS
> okay for BDRV_ZERO_OPEN to chase through layers (as long as all layers
> currently read as zero, the image reads as zero).
> 
> Upcoming patches will update the qcow2, file-posix, and nbd drivers to
> advertise the new bit when appropriate.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---

[Is it bad when I review my own patches?]

> +++ b/block.c
> @@ -5078,7 +5078,7 @@ int bdrv_known_zeroes_truncate(BlockDriverState *bs)
> 
>   int bdrv_known_zeroes(BlockDriverState *bs)
>   {
> -    int mask = BDRV_ZERO_CREATE | BDRV_ZERO_TRUNCATE;
> +    int mask = BDRV_ZERO_CREATE | BDRV_ZERO_TRUNCATE | BDRV_ZERO_OPEN;
> 
>       if (!bs->drv) {
>           return 0;
> @@ -5100,17 +5100,17 @@ int bdrv_known_zeroes(BlockDriverState *bs)
>        * ZERO_CREATE is not viable.  If the current layer is smaller
>        * than the backing layer, truncation may expose backing data,
>        * restricting ZERO_TRUNCATE; treat failure to query size in the
> -     * same manner.  Otherwise, we can trust the driver.
> +     * same manner.  For ZERO_OPEN, we insist that both backing and
> +     * current layer report the bit.
>        */
> -
>       if (bs->backing) {

Spurious line deletion caused by rebasing.


> +++ b/include/block/block.h
> @@ -105,6 +105,16 @@ typedef enum {
>        * for drivers that set .bdrv_co_truncate.
>        */
>       BDRV_ZERO_TRUNCATE      = 0x2,
> +
> +    /*
> +     * bdrv_known_zeroes() should include this bit if an image is
> +     * known to read as all zeroes when first opened; this bit should
> +     * not be relied on after any writes to the image.  This can be
> +     * set even if BDRV_ZERO_INIT is clear, but should only be set if

Rebasing snafu - I renamed that bit BDRV_ZERO_CREATE in patch 9.

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