linux-ext4 - Re: ext4 backup super block update frequency

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Message-Id: <FD2229B9-E2E1-4749-BB7E-5012E459787E@dilger.ca>
Date:   Fri, 31 Aug 2018 13:21:39 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Shehbaz Jaffer <shehbazjaffer007@...il.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: ext4 backup super block update frequency

On Aug 31, 2018, at 10:40 AM, Shehbaz Jaffer <shehbazjaffer007@...il.com> wrote:
> 
> Hi All,
> 
> I am trying to study how backup superblocks are updated and used in
> the ext4 file system. I create a 2GB file system. This creates 5
> backup superblocks with the sparse sb option. I then check the diff of
> the backup blocks before and after multiple mount, write(2) fsync(2)
> and close(2) and unmount operations.
> 
> I can see the primary superblock get updated but I do not see the
> backup super blocks being updated. My intuition is that the backup
> blocks are only present so that the recovery can be done by replaying
> the journal on the backup superblock. Are they updated each time the
> journal gets full so that the "refreshed" journal can now be replayed
> on updated backup superblock in case of a crash?
> If this is incorrect, at what frequency do backup superblocks updated?
> 
> If we compare this behavior with BtrFS, I can see that for each update
> on fs tree, the primary block (at offset 64KB) and backup superblock
> (at 64MB) gets updated.

The backup ext4 superblocks are never updated by the kernel, only after
a successful e2fsck, tune2fs, resize2fs, or other userspace operation.

This avoids clobbering the backups with bad data if the kernel has a bug
or device error (e.g. bad cable, HBA, etc).

Cheers, Andreas






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