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envelope-from=Michael.Roth@amd.com; helo=NAM12-MW2-obe.outbound.protection.outlook.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: pass (identity @amd.com) X-ZM-MESSAGEID: 1634655703461100001 From: Nir Soffer 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=3DCACHE The cache mode to be used with the file. See the documentation of the emulator's -drive cache=3D... option for allowed values. qemu(1) says: The default mode is cache=3Dwriteback. 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 fedora3= 4.img nbd+unix:///?socket=3D/tmp/nbd.sock" Benchmark #1: ./qemu-img convert -p -f raw -O raw -T none -W fedora34.i= mg nbd+unix:///?socket=3D/tmp/nbd.sock Time (mean =C2=B1 =CF=83): 83.639 s =C2=B1 5.970 s [User: 2.7= 33 s, System: 6.112 s] Range (min =E2=80=A6 max): 76.749 s =E2=80=A6 87.245 s 3 runs After this change: $ hyperfine -r3 "./qemu-img convert -p -f raw -O raw -T none -W fedora3= 4.img nbd+unix:///?socket=3D/tmp/nbd.sock" Benchmark #1: ./qemu-img convert -p -f raw -O raw -T none -W fedora34.i= mg nbd+unix:///?socket=3D/tmp/nbd.sock Time (mean =C2=B1 =CF=83): 23.522 s =C2=B1 0.433 s [User: 2.0= 83 s, System: 5.475 s] Range (min =E2=80=A6 max): 23.234 s =E2=80=A6 24.019 s 3 runs Users can avoid the issue by using --cache=3Dwriteback[1] but the defaults should give good performance for the common use case. [1] https://bugzilla.redhat.com/1990656 Signed-off-by: Nir Soffer Message-Id: <20210813205519.50518-1-nsoffer@redhat.com> Reviewed-by: Eric Blake CC: qemu-stable@nongnu.org Signed-off-by: Eric Blake (cherry picked from commit 09615257058a0ae87b837bb041f56f7312d9ead8) Signed-off-by: Michael Roth --- 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. =20 .. option:: --cache=3DCACHE =20 - The cache mode to be used with the file. See the documentation of - the emulator's ``-drive cache=3D...`` 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=3D...`` option for more info. =20 .. option:: -n, --nocache =20 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=3D[ID],snapshot.name=3D[NAME]', = or\n" " '[ID_OR_NAME]'\n" " -n, --nocache disable host cache\n" -" --cache=3DMODE set cache mode (none, writeback, ...)\n" +" --cache=3DMODE set cache mode used to access the disk imag= e, the\n" +" valid options are: 'none', 'writeback' (defau= lt),\n" +" 'writethrough', 'directsync' and 'unsafe'\n" " --aio=3DMODE set AIO mode (native, io_uring or threads)\= n" " --discard=3DMODE set discard mode (ignore, unmap)\n" " --detect-zeroes=3DMODE set detect-zeroes mode (off, on, unmap)\n" @@ -552,7 +554,7 @@ int main(int argc, char **argv) bool alloc_depth =3D false; const char *tlscredsid =3D NULL; bool imageOpts =3D false; - bool writethrough =3D true; + bool writethrough =3D false; /* Client will flush as needed. */ bool fork_process =3D false; bool list =3D false; int old_stderr =3D -1; --=20 2.25.1