On Mon, Mar 18, 2013 at 02:40:50PM +0800, Wenchao Xia wrote: > δΊ 2013-3-15 22:51, Stefan Hajnoczi ει: > > On Fri, Mar 15, 2013 at 03:24:38PM +0800, Wenchao Xia wrote: > >> I'd like to add a new way to save vmstate, which will based on the > >> migration thread, but will write contents to block images, instead > >> of fd as stream. Following is the method to add API: > > > > Hi Wenchao, > > What use cases are there besides saving vmstate to a raw image? > > > > I'm curious if you're proposing this since there is no "file:" URI or > > because you really want to do things like saving vmstate into a qcow2 > > file or over NBD. > > > > Stefan > > > Hi, Stefan > Most used cases would be "raw" and "qcow2", which is flex and can be > chosen by user. In this way, existing block layer feature in qemu can > be used, such as tagging zeros. I haven't check the buffer/cache status > in qemu block layer, but if there is, it can also benefit.
Okay, thanks for explaining. You can use caching with the BDRV_O_CACHE_WB option. Then you need to call bdrv_co_flush() to ensure data reaches the disk. The advantage of caching is that I/O patterns with many small unaligned writes may be much faster when going through the host's page cache - and reads can also be faster. You can bypass the host page cache with BDRV_O_CACHE_WB | BDRV_NO_CACHE. Here bdrv_co_flush() calls are still necessary to ensure data reaches the disk. Stefan
