linux-kernel - posix_fadvise/WILLNEED synchronous on fuse/sshfs instead of async?

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Message-ID: <20091111151937.GC20655@yumi.tdiedrich.de>
Date:	Wed, 11 Nov 2009 16:19:37 +0100
From:	Tobias Diedrich <ranma+kernel@...edrich.de>
To:	linux-kernel@...r.kernel.org
Cc:	linux-mm@...ck.org
Subject: posix_fadvise/WILLNEED synchronous on fuse/sshfs instead of async?

While trying to use posix_fadvise(...POSIX_FADVISE_WILLNEED) to
implement userspace read-ahead (with a bigger read-ahead window than
the kernel default) I found that if the underlying filesystem is
fuse/sshfs posix_fadvise is no longer doing asynchronous reads:

strace -tt with mnt/testfile on sshfs and server with very slow
upstream (ADSL):
5345  00:00:17.334209 open("mnt/testfile", O_RDONLY|O_LARGEFILE) = 3
5345  00:00:18.011383 _llseek(3, 0, [3544379], SEEK_END) = 0
5345  00:00:18.011626 _llseek(3, 0, [0], SEEK_SET) = 0
5345  00:00:18.012393 fadvise64_64(3, 0, 1048576, POSIX_FADV_WILLNEED) = 0
5345  00:01:02.438097 write(1, "[file] File size is 3544379 byte"..., 34) = 34

Note that fadvise takes 40 seconds...
Is this expected behaviour?
I would have expected that fadvise is always asynchronous and I
could rely on the call to return almost immediately.

-- 
Tobias						PGP: http://8ef7ddba.uguu.de
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