Running high IOPS against a single lun/SSD
On Solaris, and I expect the same with other operatingsystems, whenever and I/O is requested some process-level lock is set. Thismeans that if you try to run very high IOPS, this lock can become 100% busy,causing all threads that need this lock to start spinning. End result istwo-fold: high CPU utilization and/or lower than expected IOPS.
This is not a new problem. The problem was discovered severalyears ago when storage subsystems became fast enough to handle 5000 IOPS andmore. Since that time cpus have become much faster and Solaris code has beenenhanced several times to lower the need and duration for these locks. I haveseen Vdbench runs where we were able to do 100k IOPS without problems.
Vdbench is written in Java, and Java runs as a singleprocess. Vdbench therefore introduced what is called multi-JVM mode, theability of Vdbench to split the requested workload over multiple JVMs (JavaVirtual Machines).
By default Vdbench starts one JVM for each 5000 IOPS requested,with a maximum of 8, and no more than one per Storage Definition (SD). The 5000-numberprobably should be changed some day; it is a leftover of the initial discoveryof this problem.
So, when you ask for iorate=max with only a single SD andyou?re lucky enough to be running against a Solid State Device (SSD) guesswhat: you may run into this locking problem.
To work around this you have to override the default JVMcount:
[LIST] [*]Specify hd=localhost,jvms=nn I suggest you request one JVM for each 50k IOPS that you expect [*]Add ?-m nn? as an execution parameter, for instance ?-m4?. [/LIST]
There is one exception though, and that is for 100% sequentialworkloads using the seekpct=sequential or seekpct=eof Workload Definition (WD)parameter. A sequential workload will only run using one single JVM. This isdone to prevent for instance with two JVMs that the workload would look likethis: read block 1,1,2,2,3,3,4,4,5,5, etc. The performance numbers of course willlook great because the second read of a block will be guaranteed a cache hit,but this is not really a valid sequential workload.
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