Tashi
Contributor
Contributor

max LUN size

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Hi all

We ordered a new SAN from EMC. The capacity is 9TB. Now we like to create one single LUN with all the 9TB. I read something about SCSI reservation problems on big LUN`s. On this LUN should run 40 VM`s in the future with DB Applications.

Regards

Tashi

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31 Replies
RahulMM
Enthusiast
Enthusiast

Ya it might give you performance issues considering the 8 TB size you mentioned. I have not tried beyond 2 TB.....

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Tashi
Contributor
Contributor

Ok, then we will see.

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RahulMM
Enthusiast
Enthusiast

Good Luck Tashi........Share your experience furthur.....

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VforVirtualizat
Contributor
Contributor

The best way to figure out your LUN size is to calculate your throughput first and then capacity second. For example you may want to carve out a 100 GB LUN for a database that has a lot of I/O and you'll just have that one VM make use of the volume. Smaller LUN's are safer (lower risk of getting it wrong) but if you calculate your IO correctly larger LUNs are fine.

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Chuck8773
Hot Shot
Hot Shot

Correct, you can add extents to the LUN. Each LUN is limited to 2TB. Each VMFS volume can have 32 extents. So your VMFS volume can be 64 TB's. Each extent is 2TB's. Any file must reside on a single extent, so is limited to 2TB's.

Charles Killmer, VCP

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Charles Killmer, VCP4 If you found this or other information useful, please consider awarding points for "Correct" or "Helpful".
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jbogardus
Hot Shot
Hot Shot

I want to bring up Duncan's earlier comment again, because it is what will be a definate problem if/when you upgrade to vSphere later. Performance problems of creating a 8 TB are dependant on load, but the trouble you will run into with upgrading to vSphere if you create a VMFS of exactly 8 TB is a definate known. The LUNs should not be created any larger than 2 TB - 512Byte, so a VMFS datastore of 4 LUNs will be 4 TB - 2 KB.

There is a bug in ESX 3.5 that will allow you to create a VMFS of exactly 2 TB even though ESX can technically really only allow writing to 2 TB - 512Byte. vSphere now enforces the 2 TB -512B maximum properly which prevents direct upgrade of 2 TB volumes.

Reference the vSphere configuration maximums to properly plan for future upgrade when creating your datastore:

_max.pdf

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pramodupadhyay5
Enthusiast
Enthusiast

thanks for ur help ..

but i want to know y we have leave 512B and how esx server is using those 512 B....

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Tashi
Contributor
Contributor

Great thanks for all the answers. Then I will create LUN`s in size 2TB minus 512Byte, or 2047GB. And extend the VMFS to 8GB over the multiple LUN`s.

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RahulMM
Enthusiast
Enthusiast

But extent are similar to disks concatenation... so if you loose your first disk (or partition) you loose everything.

I prefer to use different datastore... or if you simple extent a LUN on storage side, you can use the new grow mode to expand also the VMFS datastore.

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Tashi
Contributor
Contributor

Interesting , I will also mention the grow mode. I`m aware that disk extents are critical when I lost one of the lun`s.

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TomHowarth
Leadership
Leadership

As Duncan stated 2TB-512Kb this will prevent issues when migrating to vSphere.

Tom Howarth VCP / VCAP / vExpert
VMware Communities User Moderator
Blog: http://www.planetvm.net
Contributing author on VMware vSphere and Virtual Infrastructure Security: Securing ESX and the Virtual Environment
Contributing author on VCP VMware Certified Professional on VSphere 4 Study Guide: Exam VCP-410
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TomHowarth
Leadership
Leadership

ESX can easily manage 256 VMFS partition and LUNs, it is not considered best practice to use extents for your VMFS partitions.

You are running the risk of data loss, if you lose a LUN from your extent you have lost the entire VMFS partition or all your VM Guests. If you have four or five separate LUNs you will only lose the Guests on that particular LUN.

The adage here is just because you can do something, does not necessarily mean it is good to do so.

Further there is no particular management overhead in having multiple LUNs attached to an ESX cluster. I would advise you to read the documentation, read the numerous posts on this site and also read some of the blog sites on VMware ESX out there.

Tom Howarth VCP / VCAP / vExpert
VMware Communities User Moderator
Blog: http://www.planetvm.net
Contributing author on VMware vSphere and Virtual Infrastructure Security: Securing ESX and the Virtual Environment
Contributing author on VCP VMware Certified Professional on VSphere 4 Study Guide: Exam VCP-410
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