cost of tdb transactions on ext4 with barriers

[Rusty Russell]

On Fri, 12 Feb 2010 11:09:45 am tridge at samba.org wrote:
> Hi Rusty,
> 
> I put a new disk in my laptop today, and thought I'd give ext4 a go
> (on a 2.6.31-19 Ubuntu karmic kernel).
> 
> I found that some of the ldb and tdb tests were massively slower on
> ext4 than ext3. I wrote a little script to demonstrate:
> 
>   http://samba.org/ftp/unpacked/junkcode/testldbspeed.sh
> 
> It creates a new fs on a spare block device, and runs a speedtest (the
> usage gives examples for tdbtorture and ldbtest).
> 
> With tdbtorture I get:
> 
>   ext3 barrier=0   1.9s
>   ext4 barrier=0   2.5s
>   ext3 barrier=1   13.4s
>   ext4 barrier=1   36.0s
> 
> The change of default in ext4 to have barriers enabled is obviously
> hurting us a lot, but what is surprising is the comparison between
> ext3 with barriers and ext4 with barriers. Why are barriers hurting
> tdb on ext4 so much more than on ext3?
> 
> I know you've recently been looking into the cost of barriers with
> tdb, so I wondered if you had any idea why this happens? 
> 
> Meanwhile, I've disabled barriers on my devel ext4 filesystem on my
> laptop as otherwise doing testing of s4 is far too painful (it adds
> about 15 minutes to 'make test').

Hmm, I don't know.  This is where we ask Ted :)

My "expertise" in this are came entirely from running tdbtorture without
barriers and pulling the plug from my test machine to demonstrate corrupted
tdbs.

Ted, you can read whole thread here:
http://lists.samba.org/archive/samba-technical/2010-February/069404.html

Cheers,
Rusty.