linux-ext4 - Re: bcache with existing ext4 filesystem

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Message-ID: <20170726200128.GB26445@amd>
Date:   Wed, 26 Jul 2017 22:01:28 +0200
From:   Pavel Machek <pavel@....cz>
To:     Eric Wheeler <bcache@...ts.ewheeler.net>
Cc:     Vojtech Pavlik <vojtech@....cz>, Theodore Ts'o <tytso@....edu>,
        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

Hi!

> > > > > > Unfortunately, that would mean shifting 400GB data 8KB forward, and
> > > > > > compatibility problems. So I'd prefer adding bcache superblock into
> > > > > > the reserved space, so I can have caching _and_ compatibility with
> > > > > > grub2 etc (and avoid 400GB move):
> > > > > 
> > > > > The common way to do that is to move the beginning of the partition,
> > > > > assuming your ext4 lives in a partition.
> > > > 
> > > > Well... if I move the partition, grub2 (etc) will be unable to access
> > > > data on it. (Plus I do not have free space before some of the
> > > > partitions I'd like to be cached).
> > > 
> > > Why not use dm-linear and prepend space for the bcache superblock?  If 
> > > this is your boot device, then you would need to write a custom 
> > > initrd hook too.
> > 
> > Thanks for a pointer. That would actually work, but I'd have to be
> > very, very careful using it...
> > 
> > ...because if I, or systemd or some kind of automounter sees the
> > underlying device (sda4) and writes to it (it is valid ext4 after
> > all), I'll have inconsistent base device and cache ... and that will
> > be asking for major problems (even in writethrough mode).
> 
> Sigh.  Gone are the days when distributions would only mount filesystems 
> if you ask them to.  
> 
> If this is a desktop, then I'm not sure what to suggest.  But for

This is a desktop.

> with no GUI, turn off the grub2 osprober (GRUB_DISABLE_OS_PROBER="true" in 
> /etc/sysconfig/grub).  If this was LVM, then I would suggest also setting 
> global_filter in lvm.conf.  If you find other places that need poked to 
> prevent automounting then please let me know!
> 
> As for ext4 feature bits, can they be arbitrarily named? (I think they are 
> bits, so maybe not).  Maybe propose a patch to ext4 to provide a

Hmm. I guess I could just set "read-only compatible" bit "this device
has a cache"... but last time I checked, ext3 still replayed log
during read-only mount...
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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