We have started seeing misaligned partitions on Linux guests runnning certain HDFS distributions. How these partitions became mis-aligned is a bit of a mystery, because the only way I know how to do this on Linux is to create a partition using old DOS format like this (using -c=dos and -u=cylinders) Continue reading “The return of misaligned IO”
Often we are presented with a vCenter screenshot, and an observation that there are “high latency spikes”. In the example, the response time is indeed quite high – around 80ms. Continue reading “High Response time and low throughput in vCenter performance charts.”
TL;DR Comparison of Paravirtual SCSI Vs Emulated SCSI in with measurements. PVSCSI gives measurably better response times at high load.
During a performance debugging session, I noticed that the response time on two of the SCSI devices was much higher than the others (Linux host under vmware ESX). The difference was unexpected since all the devices were part of the same stripe doing a uniform synthetic workload.
The immediate observation is that queue length is higher, as is wait time. All these devices reside on the same back-end storage so I am looking for something else. When I traced back the devices it turned out that the “slow devices” were attached to LSI emulated controllers in ESX. Whereas the “fast devices” are attached to para-virtual controllers.
I was surprised to see how much difference using para virtual (PV) SCSI drivers made to the guest response time once IOPS started to ramp up. In these plots the y-axis is iostat “await” time. The x-axis is time (each point is a 3 second average).
PVSCSI = Gey Dots
LSI Emulated SCSI = Red Dots
Lower is better.
Each plot is from a workload which uses a different offered IO rate. The offered rates are 8000,9000 and 10,000 the storage is able to meet the rates even though latency increases because there is a lot of outstanding IO. The workload is mixed read/write with bursts.
After converting sdh and sdi to PV SCSI the response time is again uniform across all devices.