linux-ext4 - Re: bcache with existing ext4 filesystem

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Message-ID: <20170725180225.2k3o6xtdnhygrhia@thunk.org>
Date:   Tue, 25 Jul 2017 14:02:25 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Pavel Machek <pavel@....cz>
Cc:     Reindl Harald <h.reindl@...lounge.net>, linux-ext4@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        kent.overstreet@...il.com, linux-bcache@...r.kernel.org
Subject: Re: bcache with existing ext4 filesystem

On Tue, Jul 25, 2017 at 03:46:04PM +0200, Pavel Machek wrote:
> 
> Is there some field in ext2 superblock that changes every time
> filesystem is changed? Is mtime changed by fsck/badblocks/...?

No, there isn't.  If we were writing the superblock every time the
file system is changed it would be ***extremely*** flash unfriendly.
It would also be a scalability bottleneck, it would cause us to pay an
extra HDD seek, etc.  So it's a really bad Bad BAD idea, and so we
don't do it.

Cheers,

					- Ted