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When mirroring, the goal is to ensure that the destination reads the same as the source; this goal is met whether the destination is sparse or fully-allocated (except when explicitly punching holes, then merely reading zero is not enough to know if it is sparse, so we still want to punch the hole). Avoiding a redundant write to zero (whether in the background because the zero cluster was marked in the dirty bitmap, or in the foreground because the guest is writing zeroes) when the destination already reads as zero makes mirroring faster, and avoids allocating the destination merely because the source reports as allocated. The effect is especially pronounced when the source is a raw file. That's because when the source is a qcow2 file, the dirty bitmap only visits the portions of the source that are allocated, which tend to be non-zero. But when the source is a raw file, bdrv_co_is_allocated_above() reports the entire file as allocated so mirror_dirty_init sets the entire dirty bitmap, and it is only later during mirror_iteration that we change to consulting the more precise bdrv_co_block_status_above() to learn where the source reads as zero. Remember that since a mirror operation can write a cluster more than once (every time the guest changes the source, the destination is also changed to keep up), and the guest can change whether a given cluster reads as zero, is discarded, or has non-zero data over the course of the mirror operation, we can't take the shortcut of relying on s->target_is_zero (which is static for the life of the job) in mirror_co_zero() to see if the destination is already zero, because that information may be stale. Any solution we use must be dynamic in the face of the guest writing or discarding a cluster while the mirror has been ongoing. We could just teach mirror_co_zero() to do a block_status() probe of the destination, and skip the zeroes if the destination already reads as zero, but we know from past experience that extra block_status() calls are not always cheap (tmpfs, anyone?), especially when they are random access rather than linear. Use of block_status() of the source by the background task in a linear fashion is not our bottleneck (it's a background task, after all); but since mirroring can be done while the source is actively being changed, we don't want a slow block_status() of the destination to occur on the hot path of the guest trying to do random-access writes to the source. So this patch takes a slightly different approach: any time we have to track dirty clusters, we can also track which clusters are known to read as zero. For sync=TOP or when we are punching holes from "detect-zeroes":"unmap", the zero bitmap starts out empty, but prevents a second write zero to a cluster that was already zero by an earlier pass; for sync=FULL when we are not punching holes, the zero bitmap starts out full if the destination reads as zero during initialization. Either way, I/O to the destination can now avoid redundant write zero to a cluster that already reads as zero, all without having to do a block_status() per write on the destination. With this patch, if I create a raw sparse destination file, connect it with QMP 'blockdev-add' while leaving it at the default "discard": "ignore", then run QMP 'blockdev-mirror' with "sync": "full", the destination remains sparse rather than fully allocated. Meanwhile, a destination image that is already fully allocated remains so unless it was opened with "detect-zeroes": "unmap". And any time writing zeroes is skipped, the job counters are not incremented. Signed-off-by: Eric Blake <eblake@redhat.com> Message-ID: <20250509204341.3553601-26-eblake@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> |
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| .. | ||
| export | ||
| monitor | ||
| accounting.c | ||
| aio_task.c | ||
| amend.c | ||
| backup.c | ||
| blkdebug.c | ||
| blkio.c | ||
| blklogwrites.c | ||
| blkreplay.c | ||
| blkverify.c | ||
| block-backend.c | ||
| block-copy.c | ||
| block-gen.h | ||
| block-ram-registrar.c | ||
| bochs.c | ||
| cloop.c | ||
| commit.c | ||
| copy-before-write.c | ||
| copy-before-write.h | ||
| copy-on-read.c | ||
| copy-on-read.h | ||
| coroutines.h | ||
| create.c | ||
| crypto.c | ||
| crypto.h | ||
| curl.c | ||
| dirty-bitmap.c | ||
| dmg-bz2.c | ||
| dmg-lzfse.c | ||
| dmg.c | ||
| dmg.h | ||
| file-posix.c | ||
| file-win32.c | ||
| filter-compress.c | ||
| gluster.c | ||
| graph-lock.c | ||
| io.c | ||
| io_uring.c | ||
| iscsi-opts.c | ||
| iscsi.c | ||
| linux-aio.c | ||
| meson.build | ||
| mirror.c | ||
| nbd.c | ||
| nfs.c | ||
| null.c | ||
| nvme.c | ||
| parallels-ext.c | ||
| parallels.c | ||
| parallels.h | ||
| preallocate.c | ||
| progress_meter.c | ||
| qapi-system.c | ||
| qapi.c | ||
| qcow.c | ||
| qcow2-bitmap.c | ||
| qcow2-cache.c | ||
| qcow2-cluster.c | ||
| qcow2-refcount.c | ||
| qcow2-snapshot.c | ||
| qcow2-threads.c | ||
| qcow2.c | ||
| qcow2.h | ||
| qed-check.c | ||
| qed-cluster.c | ||
| qed-l2-cache.c | ||
| qed-table.c | ||
| qed.c | ||
| qed.h | ||
| quorum.c | ||
| raw-format.c | ||
| rbd.c | ||
| replication.c | ||
| reqlist.c | ||
| snapshot-access.c | ||
| snapshot.c | ||
| ssh.c | ||
| stream.c | ||
| throttle-groups.c | ||
| throttle.c | ||
| trace-events | ||
| trace.h | ||
| vdi.c | ||
| vhdx-endian.c | ||
| vhdx-log.c | ||
| vhdx.c | ||
| vhdx.h | ||
| vmdk.c | ||
| vpc.c | ||
| vvfat.c | ||
| win32-aio.c | ||
| write-threshold.c | ||