Minor optimization for ffs/ext2fs - extra group descriptor writes

[Jaromír Doleček <jaromir.dolecek%gmail.com@localhost>]

Hi,

while poking around in ext2fs, I noticed the _all_ group description
blocks are eventually synced to disk whenever any change is made to
any of them (i.e. inode allocated/freed, or block allocated/freed), or
when there is change to superblock.

The changed group descriptors are however only written to disk on
sync, i.e. every 30 seconds or so.

There is one group description block per each ~8GB of filesystem,
depends on block size. So this is only of concern for bigger
filesystems of course.

It would be possible to optimize this to only write changed blocks,
perhaps with some kind of bitarray similar to inode/block maps, so
very little runtime overhead.

Even though it's minor optimization, I think it would still be useful
for bigger filesystems, and not hurt the small ones, so it's still
worth it to do.

Any opinion on this?

Jaromir