Re: fragmentation && blocks "realloc"

[Anton Altaparmakov <aia21@cam.ac.uk>]

On Fri, 2006-01-20 at 14:47 +0300, Jan Koss wrote:
> Hello.
> 
> Let's suppose that we have file which consist of two blocks
> and user resizing file and now we need 4 blocks.
> 
> Near this two blocks there are no 2 free blocks,
> and instead of allocating 2 additional blocks somewhere,
> I want allocate chunk of 4 blocks.
> 
> The main problem is choose way of invalidate "old" blocks and copy
> data to new buffers,
> 
> how it possible on linux?
> 
> something like
> struct buffer_head *oldbh, *newbh;
> memcpy(newbh->b_data, oldbh->b_data);
> block_invalidatepage(oldbh->b_this_page,...)

No need to invalidate or copy anything as long as you are working inside
a file system driver and those buffers are attached to page cache of a
file.

> or it is possible just change b_blocknr?

Yes, just change b_blocknr, and mark the buffer dirty so it gets written
out to the new location or indeed you can do the write (or submission
thereof) yourself if you want.

Note since you are effectively "allocating" the buffer(s), after you
have done the block allocation on your file system and updated
bh->b_blocknr, you need to call unmap_underlying_metadata(bh->b_dev,
bh->b_blocknr); for each block before you write it out or your write
could get trampled on by a different write.

And of course do not forget to deallocate the two blocks you just freed
in your fs...  (-:

Best regards,

        Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/