[DRBD-user] Read performance goes really low while writing.
Jose Ildefonso Camargo Tolosa
ildefonso.camargo at gmail.com
Thu Feb 2 02:16:54 CET 2012
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
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....
Thanks in advance,
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