Re: [Qemu-devel] [PATCH] file-posix: Cache lseek result for data regions

[Kevin Wolf <kwolf@redhat.com>]

Am 24.01.2019 um 15:40 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 24.01.2019 17:17, Kevin Wolf wrote:
> > Depending on the exact image layout and the storage backend (tmpfs is
> > konwn to have very slow SEEK_HOLE/SEEK_DATA), caching lseek results can
> > save us a lot of time e.g. during a mirror block job or qemu-img convert
> > with a fragmented source image (.bdrv_co_block_status on the protocol
> > layer can be called for every single cluster in the extreme case).
> > 
> > We may only cache data regions because of possible concurrent writers.
> > This means that we can later treat a recently punched hole as data, but
> > this is safe. We can't cache holes because then we might treat recently
> > written data as holes, which can cause corruption.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >   block/file-posix.c | 51 ++++++++++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 49 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index 8aee7a3fb8..7272c7c99d 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -168,6 +168,12 @@ typedef struct BDRVRawState {
> >       bool needs_alignment;
> >       bool check_cache_dropped;
> >   
> > +    struct seek_data_cache {
> > +        bool        valid;
> > +        uint64_t    start;
> > +        uint64_t    end;
> > +    } seek_data_cache;
> 
> Should we have some mutex-locking to protect it?

It is protected by the AioContext lock, like everything else in
BDRVRawState.

Kevin