linux-ext4 - Re: periodic lifetime_write

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Message-ID: <20221103103148.2r3gcaqpngq6jphg@fedora>
Date:   Thu, 3 Nov 2022 11:31:48 +0100
From:   Lukas Czerner <lczerner@...hat.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: periodic lifetime_write_kbytes updates?

On Wed, Nov 02, 2022 at 05:41:07PM -0600, Andreas Dilger wrote:
> I was looking at the /sys/fs/ext4/*/lifetime_write_kbytes counters on
> my home server and wondering about how accurate they are.  That is most
> interesting in the case of flash devices, to get a good idea of the
> lifetime writes vs. actual rated drive writes per day.
> 
> It looks like s_kbytes_written is only updated on clean unmount
> via ext4_commit_super->ext4_update_super() and in a few error handling
> codepaths.  This means any in-memory updates are typically lost if the
> server crashes or loses power (which is typical for long-running servers,
> rather than a clean shutdown).
> 
> It would be useful to periodically update the superblock with the current
> value, maybe once an hour if the value has changed more than some small
> margin (to take into account the *previous* update).  The superblock used
> to be written frequently via ->write_super(), but this has not been the
> case since commit v3.5-rc5-19-g4d47603d9703.
> 
> Any thoughts/objections to a periodic task calling ext4_update_super()
> every hour if there have been any noticeable writes since the last time
> it was called?  This could potentially be more clever so that it only
> writes if the disk is not asleep, and do the writes the next time it wakes,
> but I'm not sure how easy/hard that is to detect at the filesystem level.
> 
> Cheers, Andreas
> 
> PS: there is *also* a function resize.c::ext4_update_super() for added
> confusion, but that does something completely different...

Hi Andreas,

I don't have too much to contribute other than to say I think it's a
good idea. Having the counter be more precise and as such more reliable
is a good thing especially with what looks to me like a litte effort
required.

Thanks!
-Lukas