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Re: Xfs_repair and journalling -- EXT4 journal replay discussion

To: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Subject: Re: Xfs_repair and journalling -- EXT4 journal replay discussion
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Sun, 31 Mar 2013 12:35:43 +1100
Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>, Subranshu Patel <spatel.ml@xxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <5156DF72.1090703@xxxxxxxxxxxxxxxxx>
References: <CAEUQceh-Xcabr0KErxF6EAdafDDP1PY_AeHwgYB82QeUdyGp-g@xxxxxxxxxxxxxx> <5147E360.10605@xxxxxxxxxxx> <5148037B.5010706@xxxxxxxxxxxxxxxxx> <201303190924.29362.Martin@xxxxxxxxxxxx> <51483A7D.9050202@xxxxxxxxxxxxxxxxx> <5156DF72.1090703@xxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Sat, Mar 30, 2013 at 07:49:54AM -0500, Stan Hoeppner wrote:
> On 3/19/2013 5:14 AM, Stan Hoeppner wrote:
> > On 3/19/2013 3:24 AM, Martin Steigerwald wrote:
> ...
> >> Heck, even I was confused at first. Cause the manpage of fsck.ext4 IMHO is 
> >> not really clear about that topic to say the least. I tested it out for a 
> >> reason.
> > 
> > I already contacted Ted off list hoping he can point me to the relevant
> > kernel documentation, so I don't make such a mistake again with EXT.
> Ok, so here's the skinny on the source of our confusion WRT how/when
> EXT4 replays journals, and it's rather interesting.  Ted Ts'o explained
> the following.
> The EXT4 kernel module does have code to perform journal replay, but it
> is rarely executed.  The reasons for this are:
> 1.  EXT4 journal replay can take a lot of time (whereas XFS is instant)

19 minutes is my current record for XFS journal replay. 2GB log,
filled full of inode creates, required about 300,000 IOs to complete

> 2.  EXT4 systems tend to have multiple filesystems, often one per drive
>     (whereas XFS systems tend to have few filesystems)

[Citation needed]


Dave Chinner

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