[openstack-dev] [nova][cinder] Limits on volume read throughput?

Rick Jones rick.jones2 at hpe.com
Wed Mar 2 01:13:56 UTC 2016

On 03/01/2016 04:29 PM, Preston L. Bannister wrote:

> Running "dd" in the physical host against the Cinder-allocated volumes
> nets ~1.2GB/s (roughly in line with expectations for the striped flash
> volume).
> Running "dd" in an instance against the same volume (now attached to the
> instance) got ~300MB/s, which was pathetic. (I was expecting 80-90% of
> the raw host volume numbers, or better.) Upping read-ahead in the
> instance via "hdparm" boosted throughput to ~450MB/s. Much better, but
> still sad.
> In the second measure the volume data passes through iSCSI and then the
> QEMU hypervisor. I expected to lose some performance, but not more than
> half!
> Note that as this is an all-in-one OpenStack node, iSCSI is strictly
> local and not crossing a network. (I did not want network latency or
> throughput to be a concern with this first measure.)

Well, not crossing a physical network :)  You will be however likely 
crossing the loopback network on the node.

What sort of per-CPU utilizations do you see when running the test to 
the instance?  Also, out of curiosity, what block size are you using in 
dd?  I wonder how well that "maps" to what iSCSI will be doing.

rick jones

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