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 were part of the same stripe doing a uniform synthetic workload.

iostat output from the system under investigation.


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.


SuperScalin’: How I learned to stop worrying and love SQL Server on Nutanix.

TL;DR  It’s pretty easy to get 1M SQL TPM running a TPC-C like workload on a single Nutanix node.  Use 1 vDisk for Log files, and 6 vDisks for data files.  SQL Server  needs enough CPU and RAM to drive it.  I used 16 vCPU’s  and 64G of RAM.

Running database servers on Nutanix is an increasing trend and DBA’s are naturally skeptical about moving their DB’s to new platforms.  I recently had the chance to run some DB benchmarks on a couple of nodes in our lab.  My goal was to achieve 1M SQL transactions per node, and have that be linearly scalable across multiple nodes.

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It turned out to be ridiculously easy to generate decent numbers using SQL Server.  As a Unix and Oracle old-timer it was a shock to me, just how simple it is to throw up a SQL server instance.  In this experiment, I am using Windows Server 2012 and SQL-Server 2012.

For the test DB I provision 1 Disk for the SQL log files, and 6 disks for the data files.  Temp and the other system DB files are left unchanged.  Nothing is tuned or tweaked on the Nutanix side, everything is setup as per standard best practices – no “benchmark specials”.

SQL Server TPCC Scaling

Load is being generated by HammerDB configured to run the OLTP database workload.  I get a little over 1Million SQL transactions per minute (TPM) on a single Nutanix node.  The scaling is more-or-less linear, yielding 4.2 Million TPM  with 4 Nutanix nodes, which fit in a single 2U chassis . Each node is running both the DB itself, and the shared storage using NDFS.  I stopped at 6 nodes, because that’s all I had access to at the time.

The thing that blew me away in this was just how simple it had been.  Prior to using SQL server, I had been trying to set up Oracle to do the same workload.  It was a huge effort that took me back to the 1990’s, configuring kernel parameters by hand – just to stand up the DB.  I’ll come back to Oracle at a later date.

My SQL Server is configured with 16 vCPU’s and 64GB of RAM, so that the SQL server VM itself has as many resources as possible, so as not to be the bottleneck.

I use the following flags on SQL server.  In SQL terminology these are known as traceflags which are set in the SQL console (I used “DBCC trace status” to display the following.  These are fairly standard and are mentioned in our best practice guide.

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One thing I did change from the norm was to set the target recovery time to 240 seconds, rather than let SQL server determine the recovery time dynamically.  I found that in the benchmarking scenario, SQL server would not do any background flushing at all,  and then suddenly would checkpoint a huge amount of data which caused the TPM to fluctuate wildly.  With the recovery time hard coded to 240 seconds, the background page flusher keeps up with the incoming workload, and does not need to issue huge checkpoints.  My guess is that in real (non benchmark conditions) SQL server waits for the incoming work to drop-off and issues the checkpoint at that time.  Since my benchmark never backs off, SQL server eventually has to issue the checkpoint.

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Lord Kelvin Vs the IO blender

One of the characteristics of a  successful storage system for virtualized environments is that it must handle the IO blender.  Put simply, when lots of regular looking workloads are virtualized and presented to the storage, their regularity is lost, and the resulting IO stream starts to look more and more random.

 This is very similar to the way that synthesisers work – they take multiple regular sine waves of varying frequencies and add them together to get a much more complex sound.

That’s all pretty awesome for making cool space noises, but not so much when presented to the storage OS.  Without the ability to detect regularity, things like caching, pre-fetching and any kind of predictive algorithm break down.

That pre-fetch is never going to happen.

In Nutanix NOS we treat each of these sine waves (workloads) individually, never letting them get mixed together.  NDFS knows about vmdk’s or vhdx disks – and so by keeping the regular workloads separate we can still apply all the usual techniques to keep the bits flowing, even at high loads and disparate workload mixes that cause normal storage systems to fall over in a steaming heap of cache misses and metadata chaos.