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[PATCH 04/47] qemu-nbd: Change default cache mode to writeback
From: |
Michael Roth |
Subject: |
[PATCH 04/47] qemu-nbd: Change default cache mode to writeback |
Date: |
Tue, 14 Dec 2021 18:00:42 -0600 |
From: Nir Soffer <nirsof@gmail.com>
Both qemu and qemu-img use writeback cache mode by default, which is
already documented in qemu(1). qemu-nbd uses writethrough cache mode by
default, and the default cache mode is not documented.
According to the qemu-nbd(8):
--cache=CACHE
The cache mode to be used with the file. See the
documentation of the emulator's -drive cache=... option for
allowed values.
qemu(1) says:
The default mode is cache=writeback.
So users have no reason to assume that qemu-nbd is using writethough
cache mode. The only hint is the painfully slow writing when using the
defaults.
Looking in git history, it seems that qemu used writethrough in the past
to support broken guests that did not flush data properly, or could not
flush due to limitations in qemu. But qemu-nbd clients can use
NBD_CMD_FLUSH to flush data, so using writethrough does not help anyone.
Change the default cache mode to writback, and document the default and
available values properly in the online help and manual.
With this change converting image via qemu-nbd is 3.5 times faster.
$ qemu-img create dst.img 50g
$ qemu-nbd -t -f raw -k /tmp/nbd.sock dst.img
Before this change:
$ hyperfine -r3 "./qemu-img convert -p -f raw -O raw -T none -W
fedora34.img nbd+unix:///?socket=/tmp/nbd.sock"
Benchmark #1: ./qemu-img convert -p -f raw -O raw -T none -W fedora34.img
nbd+unix:///?socket=/tmp/nbd.sock
Time (mean ± σ): 83.639 s ± 5.970 s [User: 2.733 s, System: 6.112
s]
Range (min … max): 76.749 s … 87.245 s 3 runs
After this change:
$ hyperfine -r3 "./qemu-img convert -p -f raw -O raw -T none -W
fedora34.img nbd+unix:///?socket=/tmp/nbd.sock"
Benchmark #1: ./qemu-img convert -p -f raw -O raw -T none -W fedora34.img
nbd+unix:///?socket=/tmp/nbd.sock
Time (mean ± σ): 23.522 s ± 0.433 s [User: 2.083 s, System: 5.475
s]
Range (min … max): 23.234 s … 24.019 s 3 runs
Users can avoid the issue by using --cache=writeback[1] but the defaults
should give good performance for the common use case.
[1] https://bugzilla.redhat.com/1990656
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Message-Id: <20210813205519.50518-1-nsoffer@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 09615257058a0ae87b837bb041f56f7312d9ead8)
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
docs/tools/qemu-nbd.rst | 6 ++++--
qemu-nbd.c | 6 ++++--
2 files changed, 8 insertions(+), 4 deletions(-)
diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst
index ee862fa0bc..5643da26e9 100644
--- a/docs/tools/qemu-nbd.rst
+++ b/docs/tools/qemu-nbd.rst
@@ -98,8 +98,10 @@ driver options if ``--image-opts`` is specified.
.. option:: --cache=CACHE
- The cache mode to be used with the file. See the documentation of
- the emulator's ``-drive cache=...`` option for allowed values.
+ The cache mode to be used with the file. Valid values are:
+ ``none``, ``writeback`` (the default), ``writethrough``,
+ ``directsync`` and ``unsafe``. See the documentation of
+ the emulator's ``-drive cache=...`` option for more info.
.. option:: -n, --nocache
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 26ffbf15af..6c18fcd19a 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -135,7 +135,9 @@ static void usage(const char *name)
" 'snapshot.id=[ID],snapshot.name=[NAME]', or\n"
" '[ID_OR_NAME]'\n"
" -n, --nocache disable host cache\n"
-" --cache=MODE set cache mode (none, writeback, ...)\n"
+" --cache=MODE set cache mode used to access the disk image,
the\n"
+" valid options are: 'none', 'writeback'
(default),\n"
+" 'writethrough', 'directsync' and 'unsafe'\n"
" --aio=MODE set AIO mode (native, io_uring or threads)\n"
" --discard=MODE set discard mode (ignore, unmap)\n"
" --detect-zeroes=MODE set detect-zeroes mode (off, on, unmap)\n"
@@ -552,7 +554,7 @@ int main(int argc, char **argv)
bool alloc_depth = false;
const char *tlscredsid = NULL;
bool imageOpts = false;
- bool writethrough = true;
+ bool writethrough = false; /* Client will flush as needed. */
bool fork_process = false;
bool list = false;
int old_stderr = -1;
--
2.25.1
- [PATCH 39/47] nbd/server: Don't complain on certain client disconnects, (continued)
- [PATCH 39/47] nbd/server: Don't complain on certain client disconnects, Michael Roth, 2021/12/14
- [PATCH 40/47] hw/nvme: fix buffer overrun in nvme_changed_nslist (CVE-2021-3947), Michael Roth, 2021/12/14
- [PATCH 03/47] virtio-net: fix use after unmap/free for sg, Michael Roth, 2021/12/14
- [PATCH 41/47] chardev/wctable: don't free the instance in wctablet_chr_finalize, Michael Roth, 2021/12/14
- [PATCH 42/47] hw/block/fdc: Extract blk_create_empty_drive(), Michael Roth, 2021/12/14
- [PATCH 43/47] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196, Michael Roth, 2021/12/14
- [PATCH 44/47] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Michael Roth, 2021/12/14
- [PATCH 45/47] virtio-blk: Fix clean up of host notifiers for single MR transaction., Michael Roth, 2021/12/14
- [PATCH 46/47] net: vmxnet3: validate configuration values during activate (CVE-2021-20203), Michael Roth, 2021/12/14
- [PATCH 47/47] e1000: fix tx re-entrancy problem, Michael Roth, 2021/12/14
- [PATCH 04/47] qemu-nbd: Change default cache mode to writeback,
Michael Roth <=
- [PATCH 05/47] hmp: Unbreak "change vnc", Michael Roth, 2021/12/14
- [PATCH 06/47] virtio-mem-pci: Fix memory leak when creating MEMORY_DEVICE_SIZE_CHANGE event, Michael Roth, 2021/12/14
- [PATCH 07/47] uas: add stream number sanity checks., Michael Roth, 2021/12/14
- [PATCH 08/47] vhost-user: fix duplicated notifier MR init, Michael Roth, 2021/12/14
- Re: [PATCH 00/47] Patch Round-up for stable 6.1.1, freeze on 2021-12-21, Daniel P . Berrangé, 2021/12/15
- Re: [PATCH 00/47] Patch Round-up for stable 6.1.1, freeze on 2021-12-21, Michael Roth, 2021/12/20