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Date: Fri, 26 Oct 2007 16:24:46 -0500
From: Eric Sandeen <sandeen@...hat.com>
To: ext4 development <linux-ext4@...r.kernel.org>
Subject: delalloc fragmenting files?
I was doing a little seekwatchering today, and found something...
interesting.
I was doing an 8G buffered write via dd, on a machine that reports 3G of
memory, in 1M chunks like so:
dd if=/dev/zero of=/mnt/test/foobar bs=1024k count=8192
on a fairly decent hardware raid, ~90G filesystem.
Kernel is 2.6.24-rc1, with all the git patches from a day or so ago applied.
I made the ext4 fs with lustre's e2fsprogs, with -I 256, and mounted with:
mount -t ext4dev -o data=writeback,delalloc,extents,mballoc /dev/sdb7
/mnt/test
The resulting file had over 4k extents.
[root@...r-05 ~]# filefrag -v /mnt/test/foobar | grep -i extents
File is stored in extents format
/mnt/test/foobar: 4075 extents found
http://people.redhat.com/esandeen/seekwatcher/ext4-dd-write.png
if I don't mount with delalloc:
mount -t ext4dev -o data=writeback,extents,mballoc /dev/sdb7 /mnt/test
and run the same dd, I get 229 extents:
[root@...r-05 ~]# filefrag -v /mnt/test/foobar | grep -i extents
File is stored in extents format
/mnt/test/foobar: 229 extents found
http://people.redhat.com/esandeen/seekwatcher/ext4-dd-write-nodelalloc.png
It looks like delalloc is dribbling all over the disk....
(note: times & rates look wrong to me, something is up with blktrace I
think, but FIBMAP shouldn't lie about allocation)
-Eric
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