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qemu/block
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Alberto Garcia bf3d78ae55 qcow2: Stop overwriting compressed clusters one by one 
handle_alloc() tries to find as many contiguous clusters that need
copy-on-write as possible in order to allocate all of them at the same
time.

However, compressed clusters are only overwritten one by one, so let's
say that we have an image with 1024 consecutive compressed clusters:

   qemu-img create -f qcow2 hd.qcow2 64M
   for f in `seq 0 64 65472`; do
      qemu-io -c "write -c ${f}k 64k" hd.qcow2
   done

In this case trying to overwrite the whole image with one large write
request results in 1024 separate allocations:

   qemu-io -c "write 0 64M" hd.qcow2

This restriction comes from commit 095a9c58ce from 2008.
Nowadays QEMU can overwrite multiple compressed clusters just fine,
and in fact it already does: as long as the first cluster that
handle_alloc() finds is not compressed, all other compressed clusters
in the same batch will be overwritten in one go:

   qemu-img create -f qcow2 hd.qcow2 64M
   qemu-io -c "write -z 0 64k" hd.qcow2
   for f in `seq 64 64 65472`; do
      qemu-io -c "write -c ${f}k 64k" hd.qcow2
   done

Compared to the previous one, overwriting this image on my computer
goes from 8.35s down to 230ms.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: John Snow <jsnow@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2019-09-13 12:18:37 +02:00
..
accounting.c block/accounting: introduce latency histogram 2018-03-19 14:58:37 -05:00
backup.c job: drop job_drain 2019-09-10 08:58:43 +02:00
blkdebug.c blkdebug: Inject errors on .bdrv_co_block_status() 2019-06-14 14:16:57 +02:00
blklogwrites.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
blkreplay.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
blkverify.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
block-backend.c Block layer patches: 2019-08-16 16:43:46 +01:00
bochs.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
cloop.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
commit.c job: drop job_drain 2019-09-10 08:58:43 +02:00
copy-on-read.c block/copy-on-read: Fix permissions for inactive node 2019-07-30 12:25:43 +02:00
create.c block/create: Do not abort if a block driver is not available 2019-09-13 12:18:37 +02:00
crypto.c LUKS: support preallocation 2019-08-19 17:13:26 +02:00
crypto.h Clean up ill-advised or unusual header guards 2019-05-13 08:58:55 +02:00
curl.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
dirty-bitmap.c qapi: implement block-dirty-bitmap-remove transaction action 2019-08-16 16:28:03 -04:00
dmg-bz2.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
dmg-lzfse.c block: adding lzfse decompressing support as a module. 2018-12-14 11:52:40 +01:00
dmg.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
dmg.h Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
file-posix.c file-posix: Fix has_write_zeroes after NO_FALLBACK 2019-09-10 08:58:43 +02:00
file-win32.c block: Implement .bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
gluster.c block: Implement .bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
io.c block: workaround for unaligned byte range in fallocate() 2019-09-05 16:01:31 -05:00
iscsi-opts.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
iscsi.c iscsi: base all handling of check condition on scsi_sense_to_errno 2019-07-15 11:20:42 +02:00
linux-aio.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
Makefile.objs ssh: switch from libssh2 to libssh 2019-06-24 16:01:04 +02:00
mirror.c job: drop job_drain 2019-09-10 08:58:43 +02:00
nbd.c nbd: Implement client use of NBD FAST_ZERO 2019-09-05 16:03:26 -05:00
nfs.c block/nfs: add support for nfs_umount 2019-09-13 12:18:15 +02:00
null.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
nvme.c Include qemu/main-loop.h less 2019-08-16 13:31:52 +02:00
parallels.c block: Use bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
parallels.h Clean up includes 2018-02-09 05:05:11 +01:00
qapi.c qapi: add dirty-bitmaps to query-named-block-nodes result 2019-08-16 18:29:43 -04:00
qcow2-bitmap.c qcow2-bitmap: initialize bitmap directory alignment 2019-05-28 20:30:55 +02:00
qcow2-cache.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
qcow2-cluster.c qcow2: Stop overwriting compressed clusters one by one 2019-09-13 12:18:37 +02:00
qcow2-refcount.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
qcow2-snapshot.c qcow2.h: add missing include 2019-05-28 20:30:55 +02:00
qcow2-threads.c qcow2: do encryption in threads 2019-05-28 20:30:55 +02:00
qcow2.c qcow2: Fix the calculation of the maximum L2 cache size 2019-09-10 08:58:43 +02:00
qcow2.h block/qcow2: implement .bdrv_co_pwritev(_compressed)_part 2019-08-27 14:58:42 +01:00
qcow.c block/qcow: Improve error when opening qcow2 files as qcow 2019-07-08 16:00:26 +02:00
qed-check.c block/qed: add missed coroutine_fn markers 2019-04-30 15:29:00 +02:00
qed-cluster.c qed: protect table cache with CoMutex 2017-07-17 11:34:11 +08:00
qed-l2-cache.c qed: protect table cache with CoMutex 2017-07-17 11:34:11 +08:00
qed-table.c block/qed: add missed coroutine_fn markers 2019-04-30 15:29:00 +02:00
qed.c block: Implement .bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
qed.h block/qed: add missed coroutine_fn markers 2019-04-30 15:29:00 +02:00
quorum.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
raw-format.c block: Implement .bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
rbd.c block: Implement .bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
replication.c block/backup: Add mirror sync mode 'bitmap' 2019-08-16 16:28:02 -04:00
sheepdog.c block: Implement .bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
snapshot.c block/snapshot: remove bdrv_snapshot_delete_by_id_or_name 2019-02-25 15:03:18 +01:00
ssh.c block: Implement .bdrv_has_zero_init_truncate() 2019-08-19 17:13:26 +02:00
stream.c job: drop job_drain 2019-09-10 08:58:43 +02:00
throttle-groups.c Include qemu/main-loop.h less 2019-08-16 13:31:52 +02:00
throttle.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
trace-events block/backup: teach TOP to never copy unallocated regions 2019-08-16 16:28:03 -04:00
vdi.c vdi: Make block_status recurse for fixed images 2019-08-19 17:13:26 +02:00
vhdx-endian.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
vhdx-log.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
vhdx.c vhdx: Fix .bdrv_has_zero_init() 2019-08-19 17:13:26 +02:00
vhdx.h block/vhdx: Use IEC binary prefixes for size constants 2019-04-30 15:29:00 +02:00
vmdk.c vmdk: Reject invalid compressed writes 2019-09-03 14:55:35 +02:00
vpc.c vpc: Return 0 from vpc_co_create() on success 2019-09-10 08:58:43 +02:00
vvfat.c qemu-common: Move qemu_isalnum() etc. to qemu/ctype.h 2019-06-11 20:22:09 +02:00
vxhs.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
win32-aio.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
write-threshold.c qapi: Drop qapi_event_send_FOO()'s Error ** argument 2018-08-28 18:21:38 +02:00