RDO packstack very slow write on cinder volumes

asked 2017-08-16 16:02:57 -0600

laurentiusoica gravatar image

updated 2017-08-16 16:05:20 -0600

I have an RDO packstack Newton setup with 1 controller node and 2 compute nodes, all on VMs run by KVM.

The cinder volume service is on the controller node as well. It's LVM accessed with ISCSI.

The write speed on a volume attached to an instance is extremely low, ~5MB/s

#  dd if=/dev/zero of=baloon bs=1048576 count=200 oflag=dsync
200+0 records in
200+0 records out
209715200 bytes (210 MB) copied, 39.5572 s, 5.3 MB/s

The network speed between compute and storage(controller) nodes is 100MB/s

Any troubleshooting and tuning tips are appreciated.


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answered 2017-11-23 06:07:52 -0600

tshefi gravatar image


The first bottle neck I'd check would be your connection 100MB/s, 5MB/s isn't great but it won't improve by much unless you go 1G or 10G.

I'm assuming you're probably not running on SSD / fast HDD or top of the line RAM/CPU. Which is fine as it should still work but not very fast, on top of your sluggish network all of a sudden 5.3 MB/s doesn't sound that bad.

Just to put figures in perspective, found the below on some website:

*M = mega m = milli B = byte b = bit

When referring to disk usage, we measure throughput in megabytes per second, or MB/s. Notice the capital M for mega and the capital B for bytes. When referring to network performance, we measure throughput in megabits per second, or Mb/s. Notice the lowercase b. A bit is eight times smaller than a byte. You can figure out your 100Mb/s network's maximum theoretical throughput in MB simply by dividing by 8. 100 / 8 = 12.5.

TCP/IP has ~ a 10% overhead, as does Ethernet, so realistically you'll only see about 80% of that at the high end. A little more basic math shows that 12.5 * .8 = 10. You should expect to be able to write at about 10MB/s over your 100Mb/s network. This lines up perfectly with what you are seeing.*

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Asked: 2017-08-16 16:02:57 -0600

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Last updated: Nov 23 '17