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

Jose Ildefonso Camargo Tolosa ildefonso.camargo at gmail.com
Thu Feb 2 17:05:37 CET 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


On Thu, Feb 2, 2012 at 9:23 AM, Lars Ellenberg
<lars.ellenberg at linbit.com> wrote:
> 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.

I already did that, before posting to the list.  Also, note that the
problem doesn't happen if I disconnect the secondary.



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