linux-kernel - Re: Linux 2.6.29

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Date:	Mon, 30 Mar 2009 13:39:13 -0400
From:	Mark Lord <lkml@....ca>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Theodore Tso <tytso@....edu>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Jeff Garzik <jeff@...zik.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Ric Wheeler wrote:
> Linus Torvalds wrote:
..
>> That's one of the issues. The cost of those flushes can be really 
>> quite high, and as mentioned, in the absense of redundancy you don't 
>> actually get the guarantees that you seem to think that you get.
> 
> I have measured the costs of the write flushes on a variety of devices, 
> routinely, a cache flush is on the order of 10-20 ms with a healthy 
> s-ata drive.
..

Err, no.  Yes, the flush itself will be very quick,
since the drive is nearly always keeping up with the I/O
already (as we are discussing in a separate subthread here!).

But.. the cost of that FLUSH_CACHE command can be quite significant.
To issue it, we first have to stop accepting R/W requests,
and then wait for up to 32 of them currently in-flight to complete.
Then issue the cache-flush, and wait for that to complete.

Then resume R/W again.

And FLUSH_CACHE is a PIO command for most libata hosts,
so it has a multi-microsecond CPU hit as well as the I/O hit,
whereas regular R/W commands will usually use less CPU because
they are usually done via an automated host command queue.

Tiny, but significant.  And more so on smaller/slower end-user systems
like netbooks than on datacenter servers, perhaps.

Cheers
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