linux-ext4 - Re: Does ext4 write anything during reading with relatime?

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Message-ID: <20151207231913.GA3094@thunk.org>
Date:	Mon, 7 Dec 2015 18:19:13 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	The Ghost <the-ghost@...ox.ru>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Does ext4 write anything during reading with relatime?

On Tue, Dec 08, 2015 at 12:42:21AM +0300, The Ghost wrote:
> 
> I just tried again. With relatime or noatime, the second drive spins up.
> Always. With read-only, it never spins up. And that doesn't make sense...

The superblock *does* get modified at mount time to update the last
mount time and the mount count in the superblock.  But I've tested
using blktrace, and that's the only write I see after copying in a set
of test files, reading them (so that atime > mtime), and then
umounting the file system.  I then started the blktrace, mounted the
file system, and read all of the files using "tar cvzf /tmp/foo.tar.gz /mnt".

Cheers,

						- Ted
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