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Help Calculating IOPS.

Posted on 2011-02-21
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I need a little help when trying to calulate available IOPS in a raid array.  I have been looking at wmarrow's calculator (http://www.wmarow.com/strcalc/) and I am a little lost when it gets to read cache hit ratio and write cache hit ratio.  With four 7200rpm sata 3.0 drives in a raid 10 I get between 227 and 23000 IOPS (at 70%/30% read/write ratio) depending on how I manipulate these two fields.  How should I properly calculate these two fields?

I realize that in production I would want fast 15K SAS drives, and as many as possible, but this is for a lab environment and I do not have the budget for SAS drives.  Just need to get an estimate of how many iops I will have to deal with so I don't overtax the box.  It will be running vSphere Hypervisor 4.1.
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Question by:arstacey
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by:Andrew Hancock (VMware vExpert / EE MVE)
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by:Gugro
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leave the read and write cache ratio at 0.
Disks have normally only a small cache ( typical 8-32 MB ), the chance that you will hit data in this cache ( and the data was NOT found in the OS or DB cache ) is almost zero.
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driskollt earned 250 total points
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Don't factor in caching for random loads.  Eventually everything has to hit the disk so you'll take the hit somewhere.  Also, when in doubt, assume random.  Most workloads are random.  Sequentual workloads are usually things like backups, logs, data warehousing.

To calculate total sustained IOPS that a set of disks can handle for SATA w/ RAID10 and 33% writes...
A single 7200 SATA can handle about 80 sustained random IOPS.
IOPS=(80IOPS * 4disks)/(.67 + (2 * .33))
240 IOPS.
 

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by:driskollt
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Oops, sorry you wanted 70% read.

Anyway...

Total IOPS = (DriveIOPS * #Drives) / (ReadRatio + (RAIDWritePenalty * WriteRatio))
So.
246IOPS = (80 * 4) / (.7 + (2 * .3))

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by:arstacey
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Ok, so with that in mind, a RAID 10 array of 8 Sata 7200rpm 1tb drives gives me roughly 4TB of storage and 426 IOPS.  If a windows 7 desktop requires roughly 20 IOPS virtualized, then I should be able to run a Server 2008 Server Core running DC, File Shareing, DNS, and DHCP roles as well as at least 16 Win 7 desktops all virtualized under VMWare ESXi?  The server will probably only have 1 quad core xeon processor, an e5620 2.4 Ghz with Hyperthreading, and at least 24GB Ram.
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by:driskollt
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You're probably fine.  Especially if it's just a lab.  If your lab doesn't have SLAs for response time/etc, then I wouldn't worry about it too much.  

Since it's a lab, your write/read ratio is probably going to be more like .85/.15 which will give you a little better performance.

The fact you're using RAID10 with SATA for a random workload is a good idea as well.

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