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

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Date:	Thu, 26 Apr 2007 21:58:25 -0700
From:	Valerie Henson <val_henson@...ux.intel.com>
To:	Jeff Dike <jdike@...toit.com>
Cc:	Amit Gud <gud@....edu>, Nikita Danilov <nikita@...sterfs.com>,
	David Lang <david.lang@...italinsight.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	riel@...riel.com, zab@...bo.net, arjan@...radead.org,
	suparna@...ibm.com, brandon@...p.org, karunasagark@...il.com
Subject: Re: [RFC][PATCH] ChunkFS: fs fission for faster fsck

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
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