On Sat, Jan 11 2014 at 12:32pm -0500,
Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> This patch modifies dm-snapshot so that it prefetches the buffers when
> loading the exceptions.
>
> The number of buffers read ahead is specified in the DM_PREFETCH_CHUNKS
> macro.
>
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>
> ---
> drivers/md/dm-snap-persistent.c | 23 ++++++++++++++++++++++-
> 1 file changed, 22 insertions(+), 1 deletion(-)
>
> Index: linux-3.13-rc7/drivers/md/dm-snap-persistent.c
> ===================================================================
> --- linux-3.13-rc7.orig/drivers/md/dm-snap-persistent.c 2014-01-11 18:10:34.000000000 +0100
> +++ linux-3.13-rc7/drivers/md/dm-snap-persistent.c 2014-01-11 18:12:53.000000000 +0100
> @@ -18,6 +18,8 @@
> #define DM_MSG_PREFIX "persistent snapshot"
> #define DM_CHUNK_SIZE_DEFAULT_SECTORS 32 /* 16KB */
>
> +#define DM_PREFETCH_CHUNKS 12
> +
The patch header should speak to why 12 is adequate. Your 0th patch
header said we should do further testing to arrive at a good value. I
think a follow-on patch should make the 'prefetch_chunks' configurable.
The dm_exception_store structure has a 4 byte hole before the context
member. Making prefetch_chunks configurable (either from table load or
dm-snapshot module param) would help future-proof this code.
> /*-----------------------------------------------------------------
> * Persistent snapshots, by persistent we mean that the snapshot
> * will survive a reboot.
> @@ -496,6 +498,7 @@ static int read_exceptions(struct pstore
> {
> int r, full = 1;
> struct dm_bufio_client *client;
> + chunk_t prefetch_area = 0;
>
> client = dm_bufio_client_create(dm_snap_cow(ps->store->snap)->bdev,
> ps->store->chunk_size << SECTOR_SHIFT,
> @@ -504,6 +507,8 @@ static int read_exceptions(struct pstore
> if (IS_ERR(client))
> return PTR_ERR(client);
>
> + dm_bufio_set_minimum_buffers(client, DM_PREFETCH_CHUNKS + 1);
> +
> /*
> * Keeping reading chunks and inserting exceptions until
> * we find a partially full area.
> @@ -511,7 +516,23 @@ static int read_exceptions(struct pstore
> for (ps->current_area = 0; full; ps->current_area++) {
> struct dm_buffer *bp;
> void *area;
> - chunk_t chunk = area_location(ps, ps->current_area);
> + chunk_t chunk;
> +
> + if (unlikely(prefetch_area < ps->current_area))
> + prefetch_area = ps->current_area;
> +
> + if (DM_PREFETCH_CHUNKS) do {
> + chunk_t pf_chunk = area_location(ps, prefetch_area);
> + if (unlikely(pf_chunk >= dm_bufio_get_device_size(client)))
> + break;
> + if (unlikely(!dm_bufio_prefetch(client, pf_chunk, 1)))
> + break;
> + prefetch_area++;
> + if (unlikely(!prefetch_area))
> + break;
> + } while (prefetch_area <= ps->current_area + DM_PREFETCH_CHUNKS);
> +
> + chunk = area_location(ps, ps->current_area);
>
> area = dm_bufio_read(client, chunk, &bp);
> if (unlikely(IS_ERR(area))) {
>
I factored this code out to a bufio_prefetch_chunks() method, I'll share
it as v2 in reply to your original patch.
--
dm-devel mailing list
dm-devel@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/dm-devel
