linux-ext4 - Re: allowing ext4 file systems that wrapped inode count to continue working

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Date:   Mon, 30 Jul 2018 11:06:59 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Jaco Kroon <jaco@....co.za>
Cc:     Andreas Dilger <adilger@...ger.ca>, Jan Kara <jack@...e.cz>,
        linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: allowing ext4 file systems that wrapped inode count to continue
 working

On Mon, Jul 30, 2018 at 10:56:01AM +0200, Jaco Kroon wrote:
> 
> Is there any way to mark those blocks that's being freed to not be
> re-used?  I was contemplating setting them as badblocks using fsck so
> that I can online the filesystem in cycles so that I can get backups to
> function overnight, when they are done in the morning, offline and
> perform the next cycle? 

So you can use debugfs's setb, but then you can't use the allocation
bitmap to check to see whether you have accounted for all of the
groups.

If you are willing to modify and recompile the kernel, you could just
make a simple hack to ext4_mb_good_group() in fs/ext4/mballoc.c, and
add something like this at the very beginning of the funcion:

	/* replace XXX with the block group you are trying to evacuate */
	if (group == XXXX)
		return 0;

This will cause ext4 to not allocate blocks in that block group.

Similarly, instead of just specifying all of the blocks to the icheck
command, you could modify and recompile debugfs, and do something like
this at the beginning of icheck_proc():

	/* replace YYYY with the first block in the block group
	   you are trying to evacuate */
	if (*block_nr > YYYY) {
		printf("I: %lu\n", bw->inode);
		return 0;
	} 

This is super hacky since it would dedup the list of inodes, but you
can just save the output to the file, and then do something like this:

   grep -v "^I: " < debugfs.out | sed -e 's/I: //' | sort -u > /tmp/list-of-inos

Finally, a much simpler thing to do instead of copying it to the file
system you are trying to work on, is to simply copy the file somewhere
*else*.  You only need to copy the files that have blocks in the last
block group, and that's very likely less than a gig or two, so you can
probably find enough swing space on another scratch disk (even if you
have to use a USB attached HDD) as the destination.  Then you don't
need to do the hack described above to prevent allocations to that
last block group.

Good luck,

					- Ted