Re: [RFC] Ext3 online defrag

Eric Sandeen
2006-10-23T15:01:49Z

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