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

[Jamie Lokier <jamie@shareable.org>]

Armistead, Jason wrote:
> I've been following the thread about disk data consistency with some
> interest.  Given that many IDE disk drives may choose to hold data in their
> write buffers before actually writing it to disk, and given that the
> ordering of the writes may not be the same as the OS or application expects,
> the only obvious way I can see to overcome this, and ensure the data is
> truly written to the physical platters without disabling write caching is to
> overwhelm the disk drive with more data than can fit in its internal write
> buffer.
> 
> So, if you have an IDE disk with an 8Mb cache, guess what, send it an 8Mb
> chunk of random data to write out when you do an fsync().  Better still,
> locate this 8Mb as close to the middle of the travel of its heads, so that
> performance is not affected any more than necessary.  If the drive firmware
> uses a LILO or LRU policy to determine when to do its disk writes,
> overwhelming its buffers should ensure that the actual data you sent to it
> gets written out 

It doesn't work.

I thought that too, for a while, as a way to avoid sending CACHEFLUSH
commands for fs journal ordering when there is a lot of data being
written anyway.

But there is no guarantee that the drive uses a LILO or LRU policy,
and if the firmware is optimised for cache performance over a range of
benchmarks, it won't use those - there are better strategies.

You could write 8MB to the drive, but it could easily pass through the
cache without evicting some of the other data you want written.
_Especially_ if the 8MB is written to an area in the middle of the
head sweep.

> 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.

-- Jamie