Re: soft update vs journaling?

[John Richard Moser <nigelenki@comcast.net>]

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Theodore Ts'o wrote:
> On Sun, Jan 22, 2006 at 01:54:23PM -0500, John Richard Moser wrote:
> 
>>>Whenever you want to extend a filesystem to add some new feature, such
>>>as online resizing, for example, it's not enough to just add that
>>
>>Online resizing is ever safe?  I mean, with on-disk filesystem layout
>>support I could somewhat believe it for growing; for shrinking you'd
>>need a way to move files around without damaging them (possible).  I
>>guess it would be.
>>
>>So how does this work?  Move files -> alter file system superblocks?
> 
> 
> The online resizing support in ext3 only grows the filesystems; it
> doesn't shrink it.  What is currently supported in 2.6 requires you to
> reserve space in advance.  There is also a slight modification to the
> ext2/3 filesystem format which is only supported by Linux 2.6 which
> allows you to grow the filesystem without needing to move filesystem
> data structures around; the kernel patches for actualling doing this
> new style of online resizing aren't yet in mainline yet, although they
> have been posted to ext2-devel for evaluation.
> 
> 
>>A passive-active approach could passively generate a list of inodes from
>>dentries as they're accessed; and actively walk the directory tree when
>>the disk is idle.  Then a quick allocation check between inodes and
>>whatever allocation lists or trees there are could be done.
> 
> 
> That doesn't really help, because in order to release the unused disk
> blocks, you have to walk every single inode and keep track of the
> block allocation bitmaps for the entire filesystem.  If you have a
> really big filesystem, it may require hundreds of megabytes of
> non-swappable kernel memory.  And if you try to do this in userspace,
> it becomes an unholy mess trying to keep the userspace and in-kernel
> mounted filesystem data structures in sync.
> 

Yeah I figured that you couldn't take action until everything was seen;
I can see how you could have problems with all that kernel memory ;)
FUSE driver, anyone?  :>

(I've actually looked into FUSE for the rootfs, via loading a fuser
driver from an init.d and then replacing bash with init on the rootfs;
haven't found an ext2 or xfs fuse driver to test with)
> 						- Ted
> 

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