| Message ID | 1479413642-22463-3-git-send-email-eblake@redhat.com |
|---|---|
| State | New |
| Headers | show |
On 17.11.2016 21:13, Eric Blake wrote: > At the qcow2 layer, discard is only possible on a per-cluster > basis; at the moment, qcow2 silently rounds any unaligned > requests to this granularity. However, an upcoming patch will > fix a regression in the block layer ignoring too much of an > unaligned discard request, by changing the block layer to > break up a discard request at alignment boundaries; for that > to work, the block layer must know about our limits. > > However, we can't go one step further by changing > qcow2_discard_clusters() to assert that requests are always > aligned, since that helper function is reached on paths > outside of the block layer. Also, to do that, we'd have to establish that pdiscard_alignment is a mandatory alignment. Currently, it's described as the "best alignment", which doesn't sound too mandatory to me (although the block layer apparently always does drop unaligned parts). > CC: qemu-stable@nongnu.org > Signed-off-by: Eric Blake <eblake@redhat.com> > --- > block/qcow2.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/block/qcow2.c b/block/qcow2.c > index 6d5689a..e22f6dc 100644 > --- a/block/qcow2.c > +++ b/block/qcow2.c > @@ -1206,6 +1206,7 @@ static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp) > bs->bl.request_alignment = BDRV_SECTOR_SIZE; > } > bs->bl.pwrite_zeroes_alignment = s->cluster_size; > + bs->bl.pdiscard_alignment = s->cluster_size; > } > > static int qcow2_set_key(BlockDriverState *bs, const char *key) Reviewed-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/qcow2.c b/block/qcow2.c index 6d5689a..e22f6dc 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -1206,6 +1206,7 @@ static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp) bs->bl.request_alignment = BDRV_SECTOR_SIZE; } bs->bl.pwrite_zeroes_alignment = s->cluster_size; + bs->bl.pdiscard_alignment = s->cluster_size; } static int qcow2_set_key(BlockDriverState *bs, const char *key)
At the qcow2 layer, discard is only possible on a per-cluster basis; at the moment, qcow2 silently rounds any unaligned requests to this granularity. However, an upcoming patch will fix a regression in the block layer ignoring too much of an unaligned discard request, by changing the block layer to break up a discard request at alignment boundaries; for that to work, the block layer must know about our limits. However, we can't go one step further by changing qcow2_discard_clusters() to assert that requests are always aligned, since that helper function is reached on paths outside of the block layer. CC: qemu-stable@nongnu.org Signed-off-by: Eric Blake <eblake@redhat.com> --- block/qcow2.c | 1 + 1 file changed, 1 insertion(+)