[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[PATCH 49/64] qemu-nbd: Change default cache mode to writeback
From: |
Michael Roth |
Subject: |
[PATCH 49/64] qemu-nbd: Change default cache mode to writeback |
Date: |
Tue, 19 Oct 2021 09:09:29 -0500 |
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 40/64] hw/nvme: fix missing check for PMR capability, (continued)
- [PATCH 40/64] hw/nvme: fix missing check for PMR capability, Michael Roth, 2021/10/19
- [PATCH 03/64] docs/system: Document the removal of "compat" property for POWER CPUs, Michael Roth, 2021/10/19
- [PATCH 41/64] hw/nvme: fix pin-based interrupt behavior (again), Michael Roth, 2021/10/19
- [PATCH 42/64] virtio-balloon: don't start free page hinting if postcopy is possible, Michael Roth, 2021/10/19
- [PATCH 43/64] hw/net/can: sja1000 fix buff2frame_bas and buff2frame_pel when dlc is out of std CAN 8 bytes, Michael Roth, 2021/10/19
- [PATCH 44/64] hw/sd/sdcard: Document out-of-range addresses for SEND_WRITE_PROT, Michael Roth, 2021/10/19
- [PATCH 45/64] hw/sd/sdcard: Fix assertion accessing out-of-range addresses with CMD30, Michael Roth, 2021/10/19
- [PATCH 46/64] audio: Never send migration section, Michael Roth, 2021/10/19
- [PATCH 47/64] target/arm: Don't skip M-profile reset entirely in user mode, Michael Roth, 2021/10/19
- [PATCH 48/64] virtio-net: fix use after unmap/free for sg, Michael Roth, 2021/10/19
- [PATCH 49/64] qemu-nbd: Change default cache mode to writeback,
Michael Roth <=
- [PATCH 50/64] hmp: Unbreak "change vnc", Michael Roth, 2021/10/19
- [PATCH 04/64] monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB, Michael Roth, 2021/10/19
- [PATCH 51/64] virtio-mem-pci: Fix memory leak when creating MEMORY_DEVICE_SIZE_CHANGE event, Michael Roth, 2021/10/19
- [PATCH 52/64] uas: add stream number sanity checks., Michael Roth, 2021/10/19
- [PATCH 53/64] usb/redir: avoid dynamic stack allocation (CVE-2021-3527), Michael Roth, 2021/10/19
- [PATCH 54/64] usb: limit combined packets to 1 MiB (CVE-2021-3527), Michael Roth, 2021/10/19
- [PATCH 55/64] vhost-user-gpu: fix memory disclosure in virgl_cmd_get_capset_info (CVE-2021-3545), Michael Roth, 2021/10/19
- [PATCH 56/64] vhost-user-gpu: fix resource leak in 'vg_resource_create_2d' (CVE-2021-3544), Michael Roth, 2021/10/19
- [PATCH 57/64] vhost-user-gpu: fix memory leak in vg_resource_attach_backing (CVE-2021-3544), Michael Roth, 2021/10/19
- [PATCH 58/64] vhost-user-gpu: fix memory leak while calling 'vg_resource_unref' (CVE-2021-3544), Michael Roth, 2021/10/19