Re: Odd Block allocation behavior on Reiser3

[Chris Mason <mason@suse.com>]

On Mon, 2004-08-09 at 16:19, Sonny Rao wrote:
> Hi, I'm investigating filesystem performance on sequential read
> patterns of large files, and I discovered an odd pattern of block
> allocation and subsequent re-allocation after overwrite under reiser3:
> 
Exactly which kernel is this?  The block allocator in v3 has changed
recently.

> This was done on a newly created filesystem with plenty of available
> space and no other files.  I tried this test several times and saw the
> number of extents for the file vary from 5,6,7 and 134 extents, but it
> is always different after each over-write.
> 
You've hit a "feature" of the journal.  When you delete a file, the data
blocks aren't available for reuse until the transaction that allocated
them is committed to the log.  If you were to put a sync in between each
run of dd, you should get roughly the same blocks allocated each time. 
ext3 does the same things, although somewhat differently.  The
asynchronous commit is probably just finishing a little sooner on ext3.

> First, I expect that an extent-based filesystem like reiserfs

reiser4 is extent based, reiser3 is not.

-chris