Re: [Qemu-devel] Ensuring data is written to disk

[Jens Axboe]

On Tue, Aug 01 2006, Jamie Lokier wrote:
> > Of course, guessing the disk drive write buffer size and trying not to kill
> > system I/O performance with all these writes is another question entirely
> > ... sigh !!!
> 
> If you just want to evict all data from the drive's cache, and don't
> actually have other data to write, there is a CACHEFLUSH command you
> can send to the drive which will be more dependable than writing as
> much data as the cache size.

Exactly, and this is what the OS fsync() should do once the drive has
acknowledged that the data has been written (to cache). At least
reiserfs w/barriers on Linux does this.

Random write tricks are worthless, as you cannot make any assumptions
about what the drive firmware will do.

-- 
Jens Axboe