Re: [RFC] Ext3 online defrag

[Eric Sandeen <sandeen@redhat.com>]

Alex Tomas wrote:
>>>>>> Theodore Tso (TT) writes:
> 
>  TT> On Mon, Oct 23, 2006 at 02:27:10PM +0200, Jan Kara wrote:
>  >> Hello,
>  >> 
>  >> I've written a simple patch implementing ext3 ioctl for file
>  >> relocation. Basically you call ioctl on a file, give it list of blocks
>  >> and it relocates the file into given blocks (provided they are still
>  >> free). The idea is to use it as a kernel part of ext3 online
>  >> defragmenter (or generally disk access optimizer). 
> 
> isn't that a kernel responsbility to find/allocate target blocks?
> wouldn't it better to specify desirable target group and minimal
> acceptable chunk of free blocks?

XFS does this by allocating new blocks for a temporary file (initiated from 
userspace, implemented in kernelspace of course), then just checks to see if the 
result is better than what we had before; if so, then swap the storage space & 
throw away the temporary file (which now has the original, more-fragmented file 
blocks).

see xfs_swapext() in xfs_dfrag.c for the extent swapping part of this.

You probably want to avoid the page cache in all of this too, doing O_DIRECT IO 
if possible, I don't think there's any reason to churn the page cache while the 
defragmenter runs over a filesystem?

-Eric