Re: [Libguestfs] [nbdkit PATCH v2] Introduce cacheextents filter

This filter caches the last result of the extents() call and offers a nice speed-up for clients that only support req_one=1 in combination with plugins like vddk, which has no overhead for returning information for multiple extents in one call, but that call is very time-consuming. Quick test showed that on a fast connection and a sparsely allocated 16G disk with a OS installed `qemu-img map` runs 16s instead of 33s (out of which it takes 5s to the first extents request). For 100G disk with no data on it, that is one hole extent spanning the whole disk (where there is no space for improvement) this does not add any noticeable overhead. Signed-off-by: Martin Kletzander <mkletzan(a)redhat.com&gt; ---

+static int +cacheextents_fill (struct nbdkit_extents *extents, int *err) +{ + size_t i = 0; + size_t count = nbdkit_extents_count (extents); + struct nbdkit_extent first = nbdkit_get_extent (extents, 0); + struct nbdkit_extent last = nbdkit_get_extent (extents, count - 1); + + nbdkit_extents_free (cache_extents); + cache_start = first.offset; + cache_end = last.offset + last.length; + cache_extents = nbdkit_extents_new (cache_start, cache_end); + + for (i = 0; i < count; i++) { + struct nbdkit_extent ex = nbdkit_get_extent (extents, i); + nbdkit_debug ("cacheextents: updating cache with" + ": offset=%" PRIu64 + ": length=%" PRIu64 + "; type=%x", + ex.offset, ex.length, ex.type); + if (nbdkit_add_extent (cache_extents, ex.offset, ex.length, ex.type) == -1) { + *err = errno; + return -1; + }