Re: Using O_DIRECT in ext4

[Eric Sandeen <sandeen@redhat.com>]

Curt Wohlgemuth wrote:
> On Mon, Jul 20, 2009 at 8:41 PM, Eric Sandeen<sandeen@redhat.com> wrote:
>> Xiang Wang wrote:

>>> For comparison, I did the same experiment on an ext2 partition,
>>> resulting in each file having only 1 extent.
>> Interestinng, not sure I would have expected that.
> 
> Same with us; we're looking into more variables to understand it.

To be more clear, I would not have expected ext2 to deal well with it
either, is more what I meant ;)  I'm not terribly surprised that ext4
gets fragmented.

For the numbers posted, how big were the files (how many 1m chunks were
written?)

Just FWIW; I did something like:

# for I in `seq 1 16`; do dd if=/dev/zero of=testfile$I bs=1M count=16
oflag=direct & done

on a rhel5.4 beta kernel and got:

~5 extents per file on ext4 (per filefrag output)
between 41 and 234 extents on ext2.
~6 extents per file on ext3.
~16 extents per file on xfs

if I created a subdir for each file:

# for I in `seq 1 16`; do mkdir dir$I; dd if=/dev/zero
of=dir$I/testfile$I bs=1M count=16 oflag=direct & done

~5 extents per file on ext4
1 or 2 extents per file on ext2
1 or 2 extents per file on ext3
~16 extents per file on xfs.

-Eric