[DRBD-user] Read performance goes really low while writing.

[Lars Ellenberg]

On Wed, Feb 01, 2012 at 08:46:54PM -0430, Jose Ildefonso Camargo Tolosa wrote:
> Greetings,
> 
> I'm doing some experimentation with DRBD and relatively low speed
> links (~30Mbps), with latency around 34ms.  I have this:
> 
> 1. DRBD version: 8.3.7 (api:88/proto:86-91)
> 2. Several virtual machine images on the DRBD volume.
> 3. deadline scheduler by default.
> 4. LVM on top of DRBD, and ext4 on top of this LVM.
> 
> I did this:
> 
> 1. I started 3 KVM VMs (mainly idle).
> 2. I started to copy a big file (50GB) into DRBD partition.
> 3. I started to use the VMs, mainly read-only operations, with some
> small writes.
> 
> At some point, the VMs just froze while trying to read.  So, I went to
> physical machine, and tried a "ls" on a directory inside the DRBD
> volume, that I hasn't saw before (so it was not cached)... ls command
> also froze.... access to cached data seems to be unaffected, then, I
> stopped the cp process (ctrl-z), and waited for network traffic to
> stop (patiently watching bwm).  After network (replication) traffic
> stopped: all unfroze.
> 
> Now, my understanding is that DRBD reads are local, and thus: I can't
> understand why writing would freeze the reads....
> 
> Any ideas?

Try the same without DRBD, I would expect similar effects.
I suspect your are simply overloading your IO subsystem.

-- 
: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com