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From: torvalds@transmeta.com (Linus Torvalds)
Subject: Re: [POT] Which journalised filesystem ?
Date: Thu, 4 Oct 2001 23:27:45 +0000 (UTC)
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In article <E15pHJT-00041q-00@calista.inka.de>,
Bernd Eckenfels  <ecki@lina.inka.de> wrote:
>In article <Pine.LNX.4.10.10110031648250.20425-100000@coffee.psychology.mcmaster.ca> you wrote:
>> for a current Maxtor 60G 5400 RPM UDMA100 disk, 2.4.10, ext2,
>> I just measured: 7 MBps with -W0, vs 27 MB/s with -W1.
>
>how much data do you have written to get those numbers? The drive cache is
>is most often so small it only can cache a few blocks.

Actually, that's not the main win of writeback caching.

Themain win is being able to write a whole track in one go, starting at
the _right_ position (where "right" is defined as "where the head
happens to be when it can start writing). Along with making up for the
occasional seek for meta-data, and other "smooth out the writes so that
the platter keeps gettint written to all the time" things.

Which can be a HUGE win, and which is why I personally think that any
disk that doesn't do write-back caching is a waste of good money.

We (as in Linux) should make sure that we explicitly tell the disk when
we need it to flush its disk buffers. We don't do that right, and
because of _our_ problems some people claim that writeback caching is
evil and bad.

		Linus
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