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Analyze I/O Workloads to Determine Storage

Performance Requirements

 by admin

I’ve recently written this post which discusses RAID levels and how to calculate how many disks

you need in your RAID sets to provide the required performance levels for your virtual

machines. This post will look at what tools and metrics are available to analyse your virtualmachine workloads to determine how they are performing and what their storage needs are.

Analysing Disk Latency and IOPs

Disk latency is the amount of time it takes to pass an I! request from the vmkernel to the

storage array. "igh latency times occur when there are a large number of I! requests on a hostor storage subsystem. It means that the storage system is being asked to perform too many

operations# which leads to performance degradation. There are a number of tools and metrics wecan use to monitor I! work loads for latency.

vscsiStats

vscsi$tats is a tool that can be e%ecuted on a &$'i host to gather disk and latency statistics for

virtual machines. I’ve written this post that details how it can be used to help monitor storage performance. The statistics that can be collected include I! si(e# !utstanding I!s#

$eek Distance and )atency and offer more detail than those that can be gathered using es%top or

the v$phere client.

esto! and resto!

*oth es%top and res%top can be used to gatherI! latency and I!+s statistics. I’ve written aboutthis in more detail in other posts# but some of the metrics to look at include,

• "#DS/s - This is the total amount of commands per second# which includes I!+$ and

other $$I commands /e.g. reservations and locks0. 1enerally speaking 2D$s 3 I!+$

unless there are a lot of other $$I operationsmetadata operations such as reservations.

• DA$%/cmd - This is the average response time in milliseconds per command being sentto the storage device.

• &A$%/cmd - This is the amount of time the command spends in the 425ernel.

• %A$%/cmd - This is the response time as e%perienced by the 1uest !$. This is

calculated by adding together the DA41 and the 5A41 values.

http://buildvirtual.net/determine-appropriate-raid-level-for-virtual-machine-workloads/http://buildvirtual.net/determine-appropriate-raid-level-for-virtual-machine-workloads/http://buildvirtual.net/using-vscsistats-to-gather-storage-performance-data/http://buildvirtual.net/using-vscsistats-to-gather-storage-performance-data/http://buildvirtual.net/analyzing-esxtop-data/http://buildvirtual.net/using-vscsistats-to-gather-storage-performance-data/http://buildvirtual.net/analyzing-esxtop-data/http://buildvirtual.net/determine-appropriate-raid-level-for-virtual-machine-workloads/

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As a general rule DA41cmd# 5A41cmd and 1A41cmd should not e%ceed 67 milliseconds

/ms0 for sustained lengths of time.

There are also the following throughput metrics to be aware of,

•"#DS/s - As discussed above

• R'ADS/s - 8umber of read commands issued per second

• WRI('S/s - 8umber of write commands issued per second

• #)R'AD/s - 2egabytes read per second

• #)WR(*/s 9 2egabytes written per second

The sum of reads and writes equals I!+$# which is the the most common benchmark whenmonitoring and troubleshooting storage performance. These metrics can be monitored at the

"*A or 4irtual 2achine level.

vS!+ere "lient

:ou can also monitor storage performance using the v$phere client. ounters to look at includedisk read rate#disk write rate and disk usage. Disk read rate and disk write rate could be

monitored at the );8 level. *oth metrics and disk usage can be monitored per host. There a also

a number of latency counters that you can check 

Additionally# there are a handful of latency counters that should be checked# as stated here.

• deviceLatency 9 The average amount of time for the physical device to complete a $$I

command. A number greater than 6

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