n0derunner

The return of misaligned IO

Published: (Updated: ) in Debugging, , , by .

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) 

High Response time and low throughput in vCenter performance charts.

Published: (Updated: ) in Debugging, by .

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.

Impact of Paravirtual SCSI driver VS LSI Emulation with Data.

Published: (Updated: ) in Debugging, , , by . Leave a Comment on Impact of Paravirtual SCSI driver VS LSI Emulation with Data..

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 […]

Here be Zeroes

Published: (Updated: ) in Debugging, , by .

Many storage devices/filesystems treat blocks containing nothing but zeros in a special way, often short-circuiting reads from the back-end.  This is normally a good thing but this behavior can cause odd results when benchmarking.  This typically comes up when testing against storage using raw devices that have been thin provisioned. In this example, I have several […]