[DRBD-user] degraded write performace

[Jon Nelson]

On Thu, 29 Mar 2007, Duane Cox wrote:

> > Try running it longer, like 10x as long. Also, just to test it, try
> > making a single-disk software raid1 out of /dev/sdb and telling drbd to
> > use /dev/mdwhatever as it's backing store. I've seen wacky performance
> > differences depending on the "most close" block device underlying a
> > given store. Easily demonstrated with raw device access with aoe (aoe
> > will use 512b frames if you just dd to the device, whereas if you use md
> > on top of the aoe device you'll get multiples of 512b frames).
> 
> Please help me see the light...
> 
> How does the advice above have anything to do with writing to /dev/sdb 
> (which is just a single 36G 15K U160 seagate) when clearly write 
> performace to /dev/sdb degrades with each additional passing of the 
> bechmark command and only when the drbd module is loaded and not even 
> in a connnected state?
> 
> (drbd loaded in disconnected state)
> initially I get ~70 MB/s write througput to /dev/sdb
> eventually I get ~30 MB/s write througput to /dev/sdb
> possible even gets worse, I just stopped testing at this point.

^^^ I was suspecting caching. Using an additional block layer, can 
alter how and when things are cached.

> Then I STOP/UNLOAD drbd
> now I get ~70 MB/s write throughpout to /dev/sdb again.

^^^ I didn't know that when I responded.

--
Jon Nelson <jnelson-drbd at jamponi.net>