[qemu-xen stable-4.16] qemu-nbd: Change default cache mode to writeback

[patchbot@xxxxxxx]

commit 4c34ef3d341deff67bed735efca91d2ce0f9368c
Author:     Nir Soffer <nirsof@xxxxxxxxx>
AuthorDate: Fri Aug 13 23:55:19 2021 +0300
Commit:     Michael Roth <michael.roth@xxxxxxx>
CommitDate: Tue Dec 14 08:56:36 2021 -0600

    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 ± Ï?):     83.639 s ±  5.970 s    [User: 2.733 s, 
System: 6.112 s]
          Range (min â?¦ max):   76.749 s â?¦ 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 ± Ï?):     23.522 s ±  0.433 s    [User: 2.083 s, 
System: 5.475 s]
          Range (min â?¦ max):   23.234 s â?¦ 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@xxxxxxxxxx>
    Message-Id: <20210813205519.50518-1-nsoffer@xxxxxxxxxx>
    Reviewed-by: Eric Blake <eblake@xxxxxxxxxx>
    CC: qemu-stable@xxxxxxxxxx
    Signed-off-by: Eric Blake <eblake@xxxxxxxxxx>
    (cherry picked from commit 09615257058a0ae87b837bb041f56f7312d9ead8)
    Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
---
 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;
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.16