Re: [RFC][PATCH] ChunkFS: fs fission for faster fsck

[Valerie Henson <val_henson@linux.intel.com>]

On Thu, Apr 26, 2007 at 12:05:04PM -0400, Jeff Dike wrote:
> 
> No, I'm referring to a different file.  The scenario is that you have
> a growing file in a nearly full disk with files being deleted (and
> thus space being freed) such that allocations for the growing file
> bounce back and forth between chunks.

This is an excellent question.  I call this the ping-pong problem.
The solution is as Amit describes: You have a maximum of one
continuation inode per file per chunk, and you require sparse files.
Here's an example, spelled out:

Allocate file 1 in chunk A.
Grow file 1.
Chunk A fills up.
Allocate continuation inode for file 1 in chunk B.
Chunk A gets some free space.
Chunk B fills up.
Pick chunk A for allocating next block of file 1.
Try to look up a continuation inode for file 1 in chunk A.
Continuation inode for file 1 found in chunk A!
Attach newly allocated block to existing inode for file 1 in chunk A.

This is why the file format inside each chunk needs to support sparse
files.

I have a presentation that has a series of slides on problems and
potential resolutions that might help:

http://infohost.nmt.edu/~val/review/chunkfs_presentation.pdf

-VAL