On Sun, Apr 29, 2001 at 12:50:08AM -0300, Rik van Riel wrote:
> For the people wanting to turn on write caching ... it WILL break
> the write ordering needed by softupdates and journaling filesystems,
> so don't do it unless you know what you're doing.
>
> I guess it would be better to do this kind of write caching at the
> kernel level, because the OS has a much better idea of when to write
> which data to platter than a harddisk can ever have.
However, on ATA without tagged queuing, turning off write
caching (on my own UDMA33 system) reduces write performance by
a factor of two. From 12MB/s to 6MB/s on my system. That is
almost certainly because (a) ATA limits individual transfers to
64k, and (b) you can't get the next 64k into the drive before
you miss the opportunity to stream the data into the next
sector, so you lose a revolution _every_ write.
I think I'll rely on the power system and my nightly backups,
and leave .wc=1, thanks.
Sure, on my next system I'll either go back to SCSI or find some
ATA tagged queuing drives, but at the moment, I have to use what
I've got...
--
Andrew
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