Hi,
On Sat, 2005-12-10 at 13:14 -0700, Michael Loftis wrote:
> ReiserFS has hot expansion capabilities, but no (yet?) hot shrinking
> capabilities. One of the reasons it has these features and ext2/3 does not
> is because ext2/3 are very old filesystems designed on a different
> mentality of a static filesystem. On-line expansion of ext2 based
> filesystems is an extremely complicated venture, it might honestly even be
> impossible.
I guess that the code that went into fs/ext3/resize.c last year must
have been in my imagination, then. :-) And the
# lvextend -L+20g /dev/ext/backup
# ext2online /disk/backup
that I did 2 days ago to add another 20G to the mounted backup volume
must have been a dream... (I've used these same commands while the
backup was actively in progress in the past, too.)
Seriously, it's really no big deal to grow ext2/3 filesystems, with one
exception --- there's a single data structure, the group descriptor
table, that we're saddled with for backwards compatibility purposes
which needs to grow in-place when we add new block groups to the fs.
Andreas Dilger did work a while ago to add pre-allocation for that
space; mkfs with "-O resize_inode" and a new hidden inode is created
with space reserved for the group descriptor table to grow into. After
that, online resize has no trouble with ext2 on-disk format issues.
> ReiserFS has the advantage here because it doesn't necessarily pre-write
> out a lot of filesystem meta-information (superblocks, inodes, bitmaps,
> etc)
ext2/3 writes that metadata out in discrete block groups, so adding new
block groups to grow a fs is really very simple.
> Ext2 resizing requires actually rewriting a lot
> of filesystem metadata.
No; it only requires adding new metadata. Even that troublesome group
descriptor table only needs new entries added, not existing ones
modified. And once all the new metadata is written, a single write to
the superblock's number-of-block-groups field enables all the new space
atomically.
--Stephen
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