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Re: [Qemu-devel] Ensuring data is written to disk
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] Ensuring data is written to disk |
Date: |
Tue, 1 Aug 2006 11:17:44 +0100 |
User-agent: |
Mutt/1.4.1i |
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
- Re: [Qemu-devel] Ensuring data is written to disk,
Jamie Lokier <=
- Re: [Qemu-devel] Ensuring data is written to disk, Jens Axboe, 2006/08/01
- Re: [Qemu-devel] Ensuring data is written to disk, Jamie Lokier, 2006/08/01
- Re: [Qemu-devel] Ensuring data is written to disk, Jens Axboe, 2006/08/01
- Re: [Qemu-devel] Ensuring data is written to disk, Jamie Lokier, 2006/08/01
- Re: [Qemu-devel] Ensuring data is written to disk, Jens Axboe, 2006/08/02
- Re: [Qemu-devel] Ensuring data is written to disk, Jamie Lokier, 2006/08/02
- Re: [Qemu-devel] Ensuring data is written to disk, Bill C. Riemers, 2006/08/02
- Re: [Qemu-devel] Ensuring data is written to disk, R. Armiento, 2006/08/07
- Re: [Qemu-devel] Ensuring data is written to disk, Bill C. Riemers, 2006/08/07
- Re: [Qemu-devel] Ensuring data is written to disk, Thomas Steffen, 2006/08/07