linux-kernel - Re: Proposal for "proper" durable fsync() and fdatasync()

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Date:	Tue, 25 Nov 2008 10:17:14 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Sachin Gaikwad <sachin.kernel@...il.com>
Cc:	Ric Wheeler <ric@....com>, Jeff Garzik <jeff@...zik.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Chris Wedgwood <cw@...f.org>
Subject: Re: Proposal for "proper" durable fsync() and fdatasync()

Sachin Gaikwad wrote:
> > No, fsync() doesn't always flush the drive's write cache.  It often
> > does, any I think many people are under the impression it always does,
> > but it doesn't.
> >
> > Try this code on ext3:
> >
> >        fd = open ("test_file", O_RDWR | O_CREAT | O_TRUNC, 0666);
> >        while (1) {
> >                char byte;
> >                usleep (100000);
> >                pwrite (fd, &byte, 1, 0);
> >                fsync (fd);
> >        }
> >
> > It will do just over 10 write ops per second on an idle system (13 on
> > mine), and 1 flush op per second.
> 
> How did you measure write-ops and flush-ops ? Is there any tool which
> can be used ? I tried looking at what CONFIG_BSD_PROCESS_ACCT
> provides, but no luck.

I don't remember; it was such a long time ago!

It probably involved looking at /sys/block/*/stat or something like that.

You might find the "blktrace" tool does what you want.

-- Jamie
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