Re: [PATCH 1/4] block: Add bdrv_make_empty()

[Kevin Wolf]

Am 28.04.2020 um 16:07 hat Eric Blake geschrieben:
> On 4/28/20 9:01 AM, Kevin Wolf wrote:
> 
> > > Can we please fix this to take a flags parameter?  I want to make it 
> > > easier
> > > for callers to request BDRV_REQ_NO_FALLBACK for distinguishing between
> > > callers where the image must be made empty (read as all zeroes) regardless
> > > of time spent, vs. made empty quickly (including if it is already all 
> > > zero)
> > > but where the caller is prepared for the operation to fail and will write
> > > zeroes itself if fast bulk zeroing was not possible.
> > 
> > bdrv_make_empty() is not for making an image read as all zeroes, but to
> > make it fully unallocated so that the backing file becomes visible.
> > 
> > Are you confusing it with bdrv_make_zero(), which is just a wrapper
> > around bdrv_pwrite_zeroes() and does take flags?
> 
> Yes.  Although now I'm wondering if the two should remain separate or should
> just be a single driver callback where flags can include BDRV_REQ_ZERO_WRITE
> to distinguish whether exposing the backing file vs. reading as all zeroes
> is intended, or if that is merging too much.

I don't think the implementations for both things are too similar, so
you might just end up having two if branches and then two separate
implementations in the block drivers.

If anything, bdrv_make_empty() is more related to discard than
write_zeroes. But we use the discard code for it in qcow2 only as a
fallback because in the most common cases, making an image completely
empty means effectively just creating an entirely new L1 and refcount
table, which is much faster than individually discarding all clusters.

For bdrv_make_zero() I don't see an opportunity for such optimisations,
so I don't really see a reason to have a separate callback. Unless you
do know one?

Kevin