On 08/17/2018 10:04 AM, Vladimir Sementsov-Ogievskiy wrote:
Can you create a dirty bitmap with a granularity smaller than
request_alignment? I know you can configure dirty bitmap granularity
independently from cluster size (in both directions: either smaller or
larger than cluster size), but that it has a least a minimum lower
bounds of 512. You're probably right that we also want it to have a
minimum lower bound of the request_alignment (if you're using a device
with 4k minimum I/O, request_alignment would be 4k, and having a dirty
bitmap any smaller than that granularity is wasted space).
On the other hand, I also think we're safe for this patch: even if you
waste the space by creating a bitmap with too-small granularity, the
actions that write bits into the bitmap will still be aligned to
request_alignment,
Not quite right: nbd_export_bitmap searches through the backing chain
for the bitmap, and it may correspond to the bds with smaller
request_alignment.
Hmm. Interesting setup:
base (512-byte alignment, bitmap granularity) <- active (4k alignment)
so you're stating that exporting 'active' but with the 'bitmap'
inherited from the backing chain from base means that the bitmap may
have transitions in between the alignment advertised by active. Then it
sounds like nbd/server.c:bitmap_to_extents() should be aware of that
fact, and intentionally round up (anywhere that it finds a mid-alignment
transition, treat the entire aligned region as dirty. It never hurts to
mark more of the image dirty than necessary, but does hurt to expose a
mid-alignment transition to an NBD client not expecting one).
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
