linux-ext4 - Re: [PATCH] ext3: allow specifying external journal by pathname mount option

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Message-ID: <51F96964.4000608@redhat.com>
Date:	Wed, 31 Jul 2013 14:45:40 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Bill McGonigle <bill@...computing.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext3: allow specifying external journal by pathname mount
 option

On 7/31/13 12:28 PM, Bill McGonigle wrote:
> 
>> Adding a mount option, "-o journal_path=/dev/$DEVICE" would help,
>> since then we can do i.e.
>> 
>> # mount -o journal_path=/dev/disk/by-label/$JOURNAL_LABEL /mnt
> 
> I came here with a related problem, and I wonder if it would be a
> more general solution to this one too.
> 
> I've got a machine with root on ext4 on LUKS on md, and while I was
> first planning to use flashcache or dm-cache underneath it (for SSD
> acceleration) I saw benchmarks that convinced me that an external
> journal on ext4 was the better option.
> 
> It's getting it mounted at boot time that's the trick.
> 
> I saw that tune2fs supports '-J device=UUID=foo-bar-baz', so I setup
> a LUKS volume on the SSD, formatted it, passed it in, and it sets up
> the journal fine when I'm booted from external media, but when I
> reboot under the OS (3.10 in Fedora 19 in this case) it fails to
> mount because the device number has changed.
> 
> I see the proper UUID listed in 'tune2fs -l' - the "Journal UUID" is
> right, but "Journal device" is no longer correct, so boot fails until
> I remove the journal again.

Neat eh? ;)

> I think with LUKS, I'm never guaranteed
> to get a consistent device ID between boots, so the kernel command
> line options don't help either.
> 
> So, I was thinking, that if the ext code did: 1) try stored journal
> device ID 2) on fail, look up the UUID via libblkid 3) perhaps update
> the stored device ID

well, kernelspace can't use libblkid.

> it would solve my problem.  I wonder if it would be a more robust
> solution to the problem posed here as well (having the filesystem
> contain its own references is better IMHO).  My one handy system that
> has an ext3 volume (still on Fedora 16, e2fsprogs-1.41) does not show
> a Journal UUID flag via 'tune2fs -l', but I'm unaware of whether the
> flag is unavailable/missing/unsupported there or if it could be
> added.

I think my patch will solve your problems as it is, just add the new
mount option to fstab, remake your init$WHATEVER, and off you'll go.

I'll send the ext4 version in a minute if you want to try it.

-Eric

> -Bill
> 

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