Date:	Wed, 15 Jan 2014 17:28:54 +0400
From:	vitalif@...rcmc.ru
To:	linux-ext4@...r.kernel.org
Subject: A tool that allows changing inode table sizes

Hi all!

As I understand it was a well-known fact that ext2/3/4 does not allow 
changing inode table size without recreating the filesystem. And I 
didn't have any experience in linux filesystem internals until recently, 
when I've discovered that inode tables take 45 GB on one of my hard 
drives (3 TB in size) :-):-) that hard drive is, of course, full of 
movies, not 16Kb files, so the inode tables are almost 100% unused.

So, I've thought it would be good if it it would possible to change 
inode table sizes. So I've written a tool that in fact allows to do it, 
and I want to present it to the community! :)

Anyone is welcome to test it of course if it's of any interest for you - 
the source is here 
http://svn.yourcmc.ru/viewvc.py/vitalif/trunk/ext4-realloc-inodes/ 
('download tarball') (maybe it would be better to move it into a 
separate git repo, of course)

I didn't test it on a real hard drive yet :-D, only on small fs images 
with different settings (block, block group, flex_bg size, ext2/3/4, 
bigalloc and etc). There are even some auto-tests (ran by 'make test'). 
The tools works without problem on all small test images that I've 
created, though I didn't try to run it on bigger filesystems (of course 
I'll do it in the nearest future).

As this is a highly destructive process that involves overwriting ALL 
inode numbers in ALL directory entries across the whole filesystem, I've 
also implemented a simple method of safely applying/rolling back 
changes. First I've tried to use undo_io_manager, but it appears to be 
very slow because of frequent commits, which are of course needed for it 
to be safe. My method is called patch_io_manager and does a different 
thing - it does not overwrite the initial FS image, but writes all 
modified blocks into a separate sparse file + writes a bitmap of 
modified blocks in the end when it finishes. I.e. the initial filesystem 
stays unmodified.

Then, using e2patch utility (it's in the same repository), you can a) 
backup the blocks that will be modified into another patch file (e2patch 
backup <fs> <patch> <backup>) and b) apply the patch to real filesystem. 
If the applying process gets interrupted (for example by the power 
outage) it can be restarted from the beginning because it does nothing 
except just overwriting some blocks. And if the FS changes appear to be 
bad at all, you can restore the backup in a same way. So the process 
should be safe at least to some extent.

-- 
With best regards,
   Vitaliy Filippov

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