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authorNir Soffer <nirsof@gmail.com>2021-08-13 23:55:19 +0300
committerEric Blake <eblake@redhat.com>2021-09-29 13:46:31 -0500
commit09615257058a0ae87b837bb041f56f7312d9ead8 (patch)
treea02fdee78e73c66916992d2666875d587058104e
parent6b54a31bf7b403672a798b6443b1930ae6c74dea (diff)
qemu-nbd: Change default cache mode to writeback
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>
-rw-r--r--docs/tools/qemu-nbd.rst6
-rw-r--r--qemu-nbd.c6
2 files changed, 8 insertions, 4 deletions
diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst
index e39a9f4b1a..56e54cd441 100644
--- a/docs/tools/qemu-nbd.rst
+++ b/docs/tools/qemu-nbd.rst
@@ -99,8 +99,10 @@ 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 65ebec598f..9d895ba24b 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;