linux-ext4 - Re: Use case for EXT4_INODE_HUGE_FILE / EXT4_HUGE_FILE

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Date:   Tue, 7 Apr 2020 01:09:25 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: Use case for EXT4_INODE_HUGE_FILE / EXT4_HUGE_FILE_FL?

On Mon, Apr 06, 2020 at 11:30:31PM -0400, Theodore Y. Ts'o wrote:
> On Mon, Apr 06, 2020 at 03:45:34PM -0700, Josh Triplett wrote:
> > Under what circumstances can an inode ever end up with EXT4_HUGE_FILE_FL
> > set? (Other than in an artificially constructed filesystem.)
> > 
> > Was EXT4_HUGE_FILE_FL just added for future extensibility, in case a
> > future file storage mechanism allows storing files bigger than 2**32
> > blocks?
> 
> Yes. basically.  When we added the huge_file feature, which introduced
> the i_blocks_hi field, the thinking was to add EXT4_HUGE_FILE_FL so
> that we could painlessly upgrade a file system from ext3 (w/o the huge
> file feature) to enabling the feature without having to rewrite all of
> the inodes.  However, we also didn't want to artificially limit
> ourselves to 2**57 file sizes, so we also added the EXT4_HUGE_FILE_FL
> flag.

Thanks for the explanation! That makes sense.

> It hasn't gotten a huge amount of testing in a while, but it would be
> relatively easy to add debugging code (triggered via a mount option or
> a sysfs file) which forces the use of EXT4_HUGE_FILE_FL all the time.

That does seem like a good idea. It would also be nice to have an e2fsck
option to rewrite all inodes to use EXT4_HUGE_FILE_FL.

I think I'll avoid poking that code for now, though, since I don't
currently have a need for files anywhere near that large.