On Wed, Jan 19, 2011 at 10:17:47AM -0500, Chunqiang Tang wrote: > Thank both of you for the explanation, which is very helpful to me. With > FVD's capability of eliminating the host file system and storing the image > on a logical volume, then perhaps we can always use O_DSYNC, because there > is little (or no?) LVM metadata that needs a flush on every write and > hence O_DSYNC does not add overhead? I am not certain on this, and need > help for confirmation. If this is true, the guest does not need to flush > the cache.
O_DSYNC flushes the volatile write cache of the disk on every write, which can be very ineffienct. In addition to that image formats really should obey the configurable caching settings qemu has, they exist for a reason and should be handled uniformly over different image formats and protocol drivers.
