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Local storage vs SAN for VM's

Posted on 2016-10-19
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Last Modified: 2016-10-21
A typical vm(VmWare) on our network has two virtual disks; one for the OS and the other for storage.  We currently run the OS vdk(c drive) from the local drives on the host server and the D drive is a vdk on the SAN.  The SAN is a RAID 10 with 12 10K SAS drives and the host server is a RAID 10 with 8 10k SAS drives as well.  We would like to create a new RAID set on the SAN with SSD drives and move the OS vdk to it.  Would we get better performance if the SSD's were on the local host instead of the SAN?  The SAN is connected using 1Gb iSCSI.
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Question by:NytroZ
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Andrew Hancock (VMware vExpert / EE MVE) earned 250 total points
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You will probably find the bottleneck, will be the iSCSI network link to the SAN, at 1GB

Moving SSDs close to the Host CPU, will make IOPS faster.

BUT, what IOPS and performance do your require for your OS, all they do is BOOT the OS!

You may find you get better performance by moving ALL the SSDs into the HOST, and using standard spinning rust in the SAN, and using Cache-ing software.

e.g. FVP

http://pernixdata.com/
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by:andyalder
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Local storage is almost always faster than SAN assuming you have similar hardware since the same chips and algorithms used on the SAN can be used on local RAID controllers. You lose the shared storage aspect though so you can't have HA / vMotion.
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by:David Johnson, CD, MVP
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Most people that can afford a SAN have updated to 10G/40G networking to and from the SAN and other SAN's in the cluster
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by:robocat
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The only was to know if performance would be better is to measure how  much IOPS and throughput you're actually consuming currently on the C drive.

Any change will only give better performance if you're maxing out the current resources. If you're only using 10% of the currently available IO resources, then you will not see any performance difference by adding faster IO resources.

So collect some usage statistics during a normal business day and see how much IO the C drive is consuming. Most likely this will be very little which would mean that it will make little difference whatever you do, except for boot time.

The reason for moving to SAN is of course HA and failover possibilities, not performance.
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by:Gerald Connolly
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@robocat - The reason for moving to SAN is of course HA and failover possibilities, not performance.

Well actually when SAN's first came along in the late 90's one of their main advantages was performance! HA and failover came along later.
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by:robocat
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@Gerald: true if you compare SAN systems with massive cache systems and lots of spindles compared to small local RAID. In this case the SAN is similar to the local DAS.
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by:Gerald Connolly
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@robocat - was poking fun at your generalism!

Back then the debate was about big cache (e.g. EMC) and small cache (e.g Digital Storageworks), but it was mainly about performance - not all SANs then had lots of spindles, remember disks were tiny (by todays standards, think 4.4GB drives) and expensive then, and a  few hundred GB's was a BIG system!

We used to sell the concept of taking all the drives out of your server farm (lol) and consolidating them into a SAN using something like a MSA1000 or a RA9000 (including boot from SAN) to allow utilisation of the the stranded capacity not used because systems needed more spindles for performance rather than capacity - a bit like we would be now if we only had spinning rust technology (say a 1TB DB, but needing 20 spindles for performance).
Modern day SSD's and other NVM technologies have in the main, thankfully removed the IOPS issue for just about everyone.
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by:Andrew Hancock (VMware vExpert / EE MVE)
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and therefore the SAN vendors where they had a license to "print money" by selling storage because we needed IOPS by purchasing may spinning rust spindles, and we never got round to using all the storage, because we just needed IOPS

---- has long gone... because of flash!
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by:Gerald Connolly
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Didn't I just say that?
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by:Andrew Hancock (VMware vExpert / EE MVE)
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just re-enforcing how we've been ripped off by ALL Storage vendors in the past! "they've had a license to print money"

and now they are paying the price, for they're mis-trust!

and how some older storage vendors are now struggling and panicking, trying to retrofit SSD/Flash into their shelves!
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