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qemu/block
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Maxim Levitsky 867eccfed8 file-posix: Use max transfer length/segment count only for SCSI passthrough 
Regular kernel block devices (/dev/sda*, /dev/nvme*, etc) don't have
max segment size/max segment count hardware requirements exposed
to the userspace, but rather the kernel block layer
takes care to split the incoming requests that
violate these requirements.

Allowing the kernel to do the splitting allows qemu to avoid
various overheads that arise otherwise from this.

This is especially visible in nbd server,
exposing as a raw file, a mostly empty qcow2 image over the net.
In this case most of the reads by the remote user
won't even hit the underlying kernel block device,
and therefore most of the  overhead will be in the
nbd traffic which increases significantly with lower max transfer size.

In addition to that even for local block device
access the peformance improves a bit due to less
traffic between qemu and the kernel when large
transfer sizes are used (e.g for image conversion)

More info can be found at:
https://bugzilla.redhat.com/show_bug.cgi?id=1647104

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Pankaj Gupta <pagupta@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2019-07-12 15:42:23 +02:00
..
accounting.c block/accounting: introduce latency histogram 2018-03-19 14:58:37 -05:00
backup.c block: Add BlockBackend.ctx 2019-06-04 15:22:22 +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/block-backend: blk_iostatus_reset: drop usage of bs->job 2019-06-18 16:41:10 +02:00
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 block: include base when checking image chain for block allocation 2019-07-02 03:53:04 +02:00
copy-on-read.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
create.c jobs: utilize job_exit shim 2018-08-31 16:28:33 +02:00
crypto.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +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 Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
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: Use max transfer length/segment count only for SCSI passthrough 2019-07-12 15:42:23 +02:00
file-win32.c avoid TABs in files that only contain a few 2019-01-11 15:46:56 +01:00
gluster.c block/gluster: update .help of BLOCK_OPT_PREALLOC option 2019-06-12 18:32:32 +02:00
io.c block: include base when checking image chain for block allocation 2019-07-02 03:53:04 +02:00
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 Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +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 block: include base when checking image chain for block allocation 2019-07-02 03:53:04 +02:00
nbd.c block/nbd: merge NBDClientSession struct back to BDRVNBDState 2019-06-13 10:00:42 -05:00
nfs.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +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/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
parallels.c block: Add BlockBackend.ctx 2019-06-04 15:22:22 +02:00
parallels.h Clean up includes 2018-02-09 05:05:11 +01:00
qapi.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
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 Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +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: Allow -o compat=v3 during qemu-img amend 2019-07-08 16:00:31 +02:00
qcow2.h block: avoid recursive block_status call if possible 2019-06-04 15:20:41 +02:00
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-l2-cache.c
qed-table.c block/qed: add missed coroutine_fn markers 2019-04-30 15:29:00 +02:00
qed.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +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 Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
rbd.c block/rbd: increase dynamically the image size 2019-07-02 03:53:04 +02:00
replication.c block: include base when checking image chain for block allocation 2019-07-02 03:53:04 +02:00
sheepdog.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
snapshot.c block/snapshot: remove bdrv_snapshot_delete_by_id_or_name 2019-02-25 15:03:18 +01:00
ssh.c ssh: switch from libssh2 to libssh 2019-06-24 16:01:04 +02:00
stream.c block/stream: introduce a bottom node 2019-07-02 03:53:05 +02:00
throttle-groups.c throttle-groups: fix restart coroutine iothread race 2019-01-24 10:02:28 +00:00
throttle.c Include qemu/module.h where needed, drop it from qemu-common.h 2019-06-12 13:18:33 +02:00
trace-events ssh: switch from libssh2 to libssh 2019-06-24 16:01:04 +02:00
vdi.c block: Add BlockBackend.ctx 2019-06-04 15:22:22 +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 block: Add BlockBackend.ctx 2019-06-04 15:22:22 +02:00
vhdx.h block/vhdx: Use IEC binary prefixes for size constants 2019-04-30 15:29:00 +02:00
vmdk.c vmdk: Add read-only support for seSparse snapshots 2019-06-24 15:53:02 +02:00
vpc.c block: Add BlockBackend.ctx 2019-06-04 15:22:22 +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