Category: Debugging

The return of misaligned IO
By: Date: May 25, 2017 Categories: Debugging Tags: , ,

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) 

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Impact of Paravirtual SCSI driver VS LSI Emulation with Data.
By: Date: March 30, 2015 Categories: Debugging Tags: , ,

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…

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Here be Zeroes
By: Date: June 12, 2014 Categories: Debugging Tags: ,

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…

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