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

[Vladimir Sementsov-Ogievskiy]

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 <address@hidden>
> ---
>   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?

-- 
Best regards,
Vladimir