Re: [PATCH 2/2] file-posix: Cache next hole

[Max Reitz]

On 11.02.21 21:00, Eric Blake wrote:
> On 2/11/21 11:22 AM, Max Reitz wrote:
> > We have repeatedly received reports that SEEK_HOLE and SEEK_DATA are
> > slow on certain filesystems and/or under certain circumstances.  That is
> > why we generally try to avoid it (which is why bdrv_co_block_status()
> > has the @want_zero parameter, and which is why qcow2 has a metadata
> > preallocation detection, so we do not fall through to the protocol layer
> > to discover which blocks are zero, unless that is really necessary
> > (i.e., for metadata-preallocated images)).
> >
> > In addition to those measures, we can also try to speed up zero
> > detection by letting file-posix cache some hole location information,
> > namely where the next hole after the most recently queried offset is.
> > This helps especially for images that are (nearly) fully allocated,
> > which is coincidentally also the case where querying for zero
> > information cannot gain us much.
> >
> > Note that this of course only works so long as we have no concurrent
> > writers to the image, which is the case when the WRITE capability is not
> > shared.
> >
> > Alternatively (or perhaps as an improvement in the future), we could let
> > file-posix keep track of what it knows is zero and what it knows is
> > non-zero with bitmaps, which would help images that actually have a
> > significant number of holes (where this implementation here cannot do
> > much).  But for such images, SEEK_HOLE/DATA are generally faster (they
> > do not need to seek through the whole file), and the performance lost by
> > querying the block status does not feel as bad because it is outweighed
> > by the performance that can be saved by special-cases zeroed areas, so
> > focussing on images that are (nearly) fully allocated is more important.
>
> focusing

Wiktionary lists both as valid. *shrug*

> > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > ---
> >   block/file-posix.c | 81 +++++++++++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 80 insertions(+), 1 deletion(-)
>
> >   static int find_allocation(BlockDriverState *bs, off_t start,
> >                              off_t *data, off_t *hole)
> >   {
> > -#if defined SEEK_HOLE && defined SEEK_DATA
> >       BDRVRawState *s = bs->opaque;
> > +
> > +    if (s->next_zero_offset_valid) {
> > +        if (start >= s->next_zero_offset_from && start < s->next_zero_offset) {
> > +            *data = start;
> > +            *hole = s->next_zero_offset;
> > +            return 0;
> > +        }
> > +    }
> > +
> > +#if defined SEEK_HOLE && defined SEEK_DATA
>
> Why move the #if? If SEEK_HOLE is not defined, s->next_zero_offset_valid
> should never be set, because we'll treat the entire image as data.  But
> at the same time, it doesn't hurt, so doesn't stop my review.

I found it better to put it outside the SEEK_HOLE/DATA section, because 
while it won’t work when neither are defined, the code is still valid.

I don’t know. :)

> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks!

Max