Poor performance using discard

[Eric Sandeen]

On 2/28/12 10:08 PM, Dave Chinner wrote:

> Also, I think you need to provide a block trace (output of
> blktrace/blkparse for the rm -rf workloads) for both the XFS and
> ext4 cases so we can see what discards are actually being issued and
> how long they take to complete....
> 

I ran a quick test on a loopback device on 3.3.0-rc4.  Loopback supports
discards.  I made 1G filesystems on loopback on ext4 & xfs, mounted with
-o discard, cloned a git tree to them, and ran rm -rf; sync under blktrace.

XFS took about 11 seconds, ext4 took about 1.7.

(without trim, times were roughly the same - but discard/trim is probably
quite fast on the looback file)

Both files were reduced in disk usage about the same amount, so online
discard was working for both:

# du -h ext4_fsfile xfs_fsfile
497M	ext4_fsfile
491M	xfs_fsfile

XFS issued many more discards than ext4:

# blkparse xfs.trace | grep -w D | wc -l
40205
# blkparse ext4.trace | grep -w D | wc -l
123

XFS issued many small discards (4k/8 sectors) and a few larger ones:

[sectors | # discards]

8 20079
16 6762
24 3627
32 2798
40 1439
...
1840 1
7256 1
26720 1

ext4 issued far fewer discards, but in much larger chunks:

8 29
16 9
24 4
32 6
...
35152 1
35248 1
53744 1
192320 1
261624 1
262144 1

So that could certainly explain the relative speed.

-Eric