On 6/9/20 9:28 AM, Vladimir Sementsov-Ogievskiy wrote:
09.06.2020 17:08, Kevin Wolf wrote:
Since commit c8bb23cbdbe, handle_alloc_space() is called for newly
allocated clusters to efficiently initialise the COW areas with zeros if
necessary. It skips the whole operation if both start_cow nor end_cow
are empty. However, it requests zeroing the whole request size (possibly
multiple megabytes) even if only one end of the request actually needs
this.
This patch reduces the write_zeroes request size in this case so that we
don't unnecessarily zero-initialise a region that we're going to
overwrite immediately.
Hmm, I'm afraid, that this may make things worse in some cases, as with
one big write-zero request
we preallocate data-region in the protocol file, so we have better
locality for the clusters we
are going to write. And, in the same time, with BDRV_REQ_NO_FALLBACK
flag write-zero must be
fast anyway (especially in comparison with the following write request).
/*
* instead of writing zero COW buffers,
* efficiently zero out the whole clusters
*/
- ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset,
- m->nb_clusters *
s->cluster_size,
- true);
+ ret = qcow2_pre_write_overlap_check(bs, 0, start, len, true);
if (ret < 0) {
return ret;
}
BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
- ret = bdrv_co_pwrite_zeroes(s->data_file, m->alloc_offset,
- m->nb_clusters * s->cluster_size,
+ ret = bdrv_co_pwrite_zeroes(s->data_file, start, len,
BDRV_REQ_NO_FALLBACK);
Good point. If we weren't using BDRV_REQ_NO_FALLBACK, then avoiding a
pre-zero pass over the middle is essential. But since we are insisting
that the pre-zero pass be fast or else immediately fail, the time spent
in pre-zeroing should not be a concern. Do you have benchmark numbers
stating otherwise?
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
