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Lunev" Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, Vladimir Sementsov-Ogievskiy , John Snow , Andrey Drobyshev Subject: Re: [PATCH v4 12/12] block/block-copy: coalesce write-zeroes tasks Message-ID: References: <20261030010613.1612771-1-den@openvz.org> <20261030010613.1612771-13-den@openvz.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261030010613.1612771-13-den@openvz.org> User-Agent: NeoMutt/20260616 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Archive: List-Post: List-Help: List-Subscribe: , Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Fri, Oct 30, 2026 at 02:06:12AM +0100, Denis V. Lunev wrote: > From: Denis V. Lunev > > Task size was capped to block_copy_chunk_size() regardless of method, > which sizes a task by the copy buffer. A write-zeroes task carries no > buffer, so that cap only splits one long known-zero run into a crowd of > small COPY_WRITE_ZEROES tasks, each with its own request against the > target. > > block_copy_task_create() already decides from zero_bitmap how far a task > may run, so add block_copy_widen_zero_area() to that decision: a > write-zeroes task runs to the next clean cluster of copy_bitmap, at most > BDRV_REQUEST_MAX_BYTES aligned down to cluster_size, and the existing > clamp cuts it back to where the zero run ends. The experimental x-perf > max-chunk does not bound it, and its documentation now says so. > Widening is safe: an overlapping caller waits on the task's BlockReq via > reqlist_wait_one() rather than observing it mid-flight. > > A request that large is only cheap where the target zeroes by metadata. > BDRV_REQ_NO_FALLBACK in supported_zero_flags rules out the targets which > cannot, qcow2 v2 and iscsi among them, but it is optimistic for the > rest: file-posix advertises it at open and only learns from a failing > fallocate. So a write-zeroes task asks for it while the target is > believed to oblige, which costs nothing when it does. A refusal ends the > widening and shrinks the task to one cluster, written with fallback > allowed. The rest of the range is dirty again, and the next pass of > block_copy_common() picks it up as buffer-sized tasks running in > parallel. Part of the refused range may already be zeroed, since > -ENOTSUP only says the fast path is unavailable, and writing zeroes over > it again changes nothing. > > Such a task is also the only one sure to need no buffer: without > NO_FALLBACK, bdrv_co_do_pwrite_zeroes() writes the zeroes out through a > bounce buffer. So it alone is left out of the memory pool and of the > rate limit, where one widened request would otherwise hold the job back > for as long as speed= takes to pass its whole length. The one-cluster > write left after a refusal is not charged either, though it may bounce: > that is at most one cluster per worker, once, as tasks created after the > refusal are charged in full. > > Signed-off-by: Denis V. Lunev > CC: Vladimir Sementsov-Ogievskiy > CC: John Snow > CC: Andrey Drobyshev > --- > block/block-copy.c | 96 +++++++++++++++++++++++++++++++++++++++----- > qapi/block-core.json | 4 +- > 2 files changed, 88 insertions(+), 12 deletions(-) Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org