linux-ext4 - Re: Mentor for a GSoC application wanted (Online ext2/3 filesystem checker)

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Message-ID: <480CD18F.7000108@firstfloor.org>
Date:	Mon, 21 Apr 2008 19:40:31 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Ricardo M. Correia" <Ricardo.M.Correia@....COM>
CC:	Theodore Tso <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>,
	Alexey Zaytsev <alexey.zaytsev@...il.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Rik van Riel <riel@...riel.com>
Subject: Re: Mentor for a GSoC application wanted (Online ext2/3 filesystem
 checker)

> Am I correct that the Linux fsync(), when used (from userspace)
> directly on file descriptors associated with block devices doesn't
> actually flush the disk write cache and wait for the data to reach the
> disk before returning?

Not quite. It depends. Sometimes it does this and sometimes it doesn't,
depending on the disk and the controller and the file system and the
kernel version and the distribution default.

For details search the archives of linux-kernel/linux-fsdevel. This
has been discussed many times.

> Is there a reason why this isn't being done other than performance?

One reason against it is that in many (but not all) setups to guarantee
reaching the platter you have to disable the write cache, and at least
for consumer level hard disks disk vendors generally do not recommend
doing this because it significantly lowers the MTBF of the disk.

-Andi

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