I have need to benchmark volume-read performance of an application running in an instance, assuming extremely fast storage.
To simulate fast storage, I have an AIO install of OpenStack, with local flash disks. Cinder LVM volumes are striped across three flash drives (what I have in the present setup). Since I am only interested in sequential-read performance, the "dd" utility is sufficient as a measure. 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.) I do not see any prior mention of performance of this sort on the web or in the mailing list. Possible I missed something. What sort of numbers are you seeing out of high performance storage? Is the *huge* drop in read-rate within an instance something others have seen?
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