113 lines
4.6 KiB
Diff
113 lines
4.6 KiB
Diff
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From: Nir Soffer <nirsof@gmail.com>
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Date: Fri, 13 Aug 2021 23:55:19 +0300
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Subject: qemu-nbd: Change default cache mode to writeback
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Git-commit: 09615257058a0ae87b837bb041f56f7312d9ead8
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Both qemu and qemu-img use writeback cache mode by default, which is
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already documented in qemu(1). qemu-nbd uses writethrough cache mode by
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default, and the default cache mode is not documented.
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According to the qemu-nbd(8):
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--cache=CACHE
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The cache mode to be used with the file. See the
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documentation of the emulator's -drive cache=... option for
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allowed values.
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qemu(1) says:
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The default mode is cache=writeback.
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So users have no reason to assume that qemu-nbd is using writethough
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cache mode. The only hint is the painfully slow writing when using the
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defaults.
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Looking in git history, it seems that qemu used writethrough in the past
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to support broken guests that did not flush data properly, or could not
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flush due to limitations in qemu. But qemu-nbd clients can use
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NBD_CMD_FLUSH to flush data, so using writethrough does not help anyone.
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Change the default cache mode to writback, and document the default and
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available values properly in the online help and manual.
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With this change converting image via qemu-nbd is 3.5 times faster.
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$ qemu-img create dst.img 50g
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$ qemu-nbd -t -f raw -k /tmp/nbd.sock dst.img
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Before this change:
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$ hyperfine -r3 "./qemu-img convert -p -f raw -O raw -T none -W fedora34.img nbd+unix:///?socket=/tmp/nbd.sock"
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Benchmark #1: ./qemu-img convert -p -f raw -O raw -T none -W fedora34.img nbd+unix:///?socket=/tmp/nbd.sock
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Time (mean ± σ): 83.639 s ± 5.970 s [User: 2.733 s, System: 6.112 s]
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Range (min … max): 76.749 s … 87.245 s 3 runs
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After this change:
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$ hyperfine -r3 "./qemu-img convert -p -f raw -O raw -T none -W fedora34.img nbd+unix:///?socket=/tmp/nbd.sock"
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Benchmark #1: ./qemu-img convert -p -f raw -O raw -T none -W fedora34.img nbd+unix:///?socket=/tmp/nbd.sock
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Time (mean ± σ): 23.522 s ± 0.433 s [User: 2.083 s, System: 5.475 s]
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Range (min … max): 23.234 s … 24.019 s 3 runs
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Users can avoid the issue by using --cache=writeback[1] but the defaults
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should give good performance for the common use case.
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[1] https://bugzilla.redhat.com/1990656
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Signed-off-by: Nir Soffer <nsoffer@redhat.com>
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Message-Id: <20210813205519.50518-1-nsoffer@redhat.com>
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Reviewed-by: Eric Blake <eblake@redhat.com>
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CC: qemu-stable@nongnu.org
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Signed-off-by: Eric Blake <eblake@redhat.com>
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Signed-off-by: Li Zhang <li.zhang@suse.com>
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---
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docs/tools/qemu-nbd.rst | 6 ++++--
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qemu-nbd.c | 6 ++++--
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2 files changed, 8 insertions(+), 4 deletions(-)
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diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst
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index ee862fa0bc02667bb67f99447b23..5643da26e98241c1fa0969b90b2c 100644
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--- a/docs/tools/qemu-nbd.rst
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+++ b/docs/tools/qemu-nbd.rst
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@@ -98,8 +98,10 @@ driver options if ``--image-opts`` is specified.
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.. option:: --cache=CACHE
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- The cache mode to be used with the file. See the documentation of
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- the emulator's ``-drive cache=...`` option for allowed values.
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+ The cache mode to be used with the file. Valid values are:
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+ ``none``, ``writeback`` (the default), ``writethrough``,
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+ ``directsync`` and ``unsafe``. See the documentation of
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+ the emulator's ``-drive cache=...`` option for more info.
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.. option:: -n, --nocache
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diff --git a/qemu-nbd.c b/qemu-nbd.c
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index 26ffbf15af0a755dddc99e27c876..6c18fcd19a07b7194a5c2defdc73 100644
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--- a/qemu-nbd.c
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+++ b/qemu-nbd.c
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@@ -135,7 +135,9 @@ static void usage(const char *name)
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" 'snapshot.id=[ID],snapshot.name=[NAME]', or\n"
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" '[ID_OR_NAME]'\n"
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" -n, --nocache disable host cache\n"
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-" --cache=MODE set cache mode (none, writeback, ...)\n"
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+" --cache=MODE set cache mode used to access the disk image, the\n"
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+" valid options are: 'none', 'writeback' (default),\n"
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+" 'writethrough', 'directsync' and 'unsafe'\n"
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" --aio=MODE set AIO mode (native, io_uring or threads)\n"
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" --discard=MODE set discard mode (ignore, unmap)\n"
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" --detect-zeroes=MODE set detect-zeroes mode (off, on, unmap)\n"
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@@ -552,7 +554,7 @@ int main(int argc, char **argv)
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bool alloc_depth = false;
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const char *tlscredsid = NULL;
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bool imageOpts = false;
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- bool writethrough = true;
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+ bool writethrough = false; /* Client will flush as needed. */
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bool fork_process = false;
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bool list = false;
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int old_stderr = -1;
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