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

[Jose Ildefonso Camargo Tolosa]

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?

Thanks in advance,

Ildefonso Camargo