In the previous discussion Anthony brought up the fact that cache=writeback is safe enough considering data integrity. If so, I don't see a reason not to use it as default, as it speeds up things a lot.
Keep in mind that most people will want to use cache=none anyways if they want to bypass the host cache. To stress my point, I ran the same test I did for cache=unsafe again to compare cache=write{through,back}. cache=writeback real 0m32.549s user 0m16.069s sys 0m6.152s cache=writethrough real 0m48.055s user 0m16.677s sys 0m8.045s Signed-off-by: Alexander Graf <ag...@suse.de> --- qemu-options.hx | 19 +++++++++---------- vl.c | 5 ++++- 2 files changed, 13 insertions(+), 11 deletions(-) diff --git a/qemu-options.hx b/qemu-options.hx index cea9b72..b389f36 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -162,16 +162,15 @@ This option specifies the serial number to assign to the device. Specify the controller's PCI address (if=virtio only). @end table -By default, writethrough caching is used for all block device. This means that -the host page cache will be used to read and write data but write notification -will be sent to the guest only when the data has been reported as written by -the storage subsystem. - -Writeback caching will report data writes as completed as soon as the data is -present in the host page cache. This is safe as long as you trust your host. -If your host crashes or loses power, then the guest may experience data -corruption. When using the @option{-snapshot} option, writeback caching is -used by default. +By default, writeback caching is used for all block devices. It will report +data writes as completed as soon as the data is present in the host page +cache. This is safe as long as you trust your host. If your host crashes +or loses power, then the guest may experience data corruption. When using +the @option{-snapshot} option, writeback caching is always used. + +Writethrough caching means that the host page cache will be used to read +and write data but write notification will be sent to the guest only when +the data has been reported as written by the storage subsystem. The host page cache can be avoided entirely with @option{cache=none}. This will attempt to do disk IO directly to the guests memory. QEMU may still perform diff --git a/vl.c b/vl.c index bc15dd7..a70e487 100644 --- a/vl.c +++ b/vl.c @@ -917,11 +917,14 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque, bdrv_flags |= BDRV_O_CACHE_WB; bdrv_flags |= BDRV_O_NO_FLUSH; } else if (!strcmp(buf, "writethrough")) { - /* this is the default */ + /* no special flag needed */ } else { fprintf(stderr, "qemu: invalid cache option\n"); return NULL; } + } else { + /* default to writeback */ + bdrv_flags |= BDRV_O_CACHE_WB; } #ifdef CONFIG_LINUX_AIO -- 1.6.0.2