LUN Size

[Thiago_Moreira]

Hi all.

I´m a new ESX/user and I have a simple doubt... Some days ago a VCP told to me that large LUNs impacts on ESX performance. So, what is the maximum size of a storage LUN to put my vm files?

Today I have two LUNs: one for vm system files, and other to hold vmdk of data (host D:\ drive, most cases). I´m using a Dell MD3000i iSCSI storage.

Suggestions?

[Rubeck]

The maximun size is 2TB per VMFS LUN

And yes, smaller LUNs perform better, althoug I do not think this is very noticable.

Our main reason for creating smaller LUNs (around 500GB) is to limit the number of .vmdk files placed on each. To many VMs accessing .vmdks on the same LUN will decrease disk performance.... All this depends on the storage system used ofc.

/Rubeck

[madda]

The maximum LUN size is 2048GB with

When you format the LUN with VMFS you get the choice of block size, which determines the max size of that VMFS, below are the options:

Blocksize - Max VMFS size

1MB - 256GB

2MB - 512GB

4MB - 1024GB

8MB - 2048GB

From what I have seen there isn't any noticeable performance degradation with large LUNs over smaller LUNs from ESX's perspective.

[Thiago_Moreira]

I guess that you made a mistake, because blocksize will determine max file size, not LUN size.

In my case I have a LUN with 800 GB and blocksize of 1 MB.

[madda]

This is VMFS partition size limitations, not LUN size

It is recommended to only use only 1 LUN for 1 VMFS partition

[Rubeck]

The numbers apply to file sizes.... See: http://www.vmware.com/pdf/vi3_35/esx_3/r35/vi3_35_25_config_max.pdf

/Rubeck

[Thiago_Moreira]

Ok. 1 LUN -> 1 VMFS

VMFS max size is 2 TB. OK.

But what is the best practices?

[madda]

Have a read of the document rubeck posted, it will tell you the maximum. The max is 2TB for a VMFS LUN on fibre channel, I am reasonably sure the same applies to iSCSI although there isn't anything in there specifically about iSCSI.

I normally have 1 LUN for each VM as that means the sizes are kept below the max and there aren't multiple VMs stressing that part of the SAN at the same time.

[mike_laspina]

Hello,

The VCP you spoke with is giving you good advise.

The issue he is addressing is called reservation conflicts. It happens when too many SCSI commands are hitting a shared SCSI LUN target. Each time a server needs to read or write it makes a reservation for that resource and is its busy it return a reservation conflict and waits for the time out.

[malaysiavm]

I will suggest to keep the size within 300 GB to 500GB for your LUN. By the way, you do need to work on the SAN level to ensure you have the right raid group configuration with the spindle.

Malaysia VMware Communities - http://www.malaysiavm.com

[Rubeck]

Mike.. Does the host really make SCSI reservations for every read and write? I thought it was only during update of LUN meta data.. like when extending, writing and deleting files on the VMFS? (ofc hba rescans etc. also do reservations)

/Rubeck

[mike_laspina]

Conflicts mostly occur on shared objects data blocks such as the vmfs metadata files and can also be on vmdks in multiple ways like backups and hot cloning etc. Its a SCSI command in a multi initiator to target environment that cannot make a SCSI reservation due to a resource use conflict and it returns conflict and the operation waits. It's bad news for performance.

The LUN size is really a matter of working out what will be on the LUN in terms of VMs

VM's with lots of memory could add up to like of 15 * 3G = 45G of swap file

If you have 16G system disks you now have 15 * 16G = 240G

And then some data volumes 3 * 50 = 150G

Well we are already at 435G

Throw some snapshot and template activity and it's at 500G minimum.

[Rubeck]

Agree with you all the way...:-)

Sorry, , I just seem to have misread your previous post :smileycool:

[Thiago_Moreira]

So, as a general rule I would store 10 ~15 vmdk on the same LUN.....

If each file have 50 GB, a LUN/VMFS with 1.5 TB will have same performance of 3 LUNs with 500 GB.

Thats correct?

[mike_laspina]

Essentially yes. There are some caviats where if the I/O type is very small performance would be better on the smaller LUN because the number of commands would be elevated.

For example a File and printer server will have large bursty transfers while a DB server will have frequent small tranfers.

So if an ESX server was running 15 DB VM's it would perform poorly with this type of I/O because the number of SCSI reservation requests is high.

[Thiago_Moreira]

I have a small environment with not more than 15 servers...

one DB

one file server

one mail server

web server

etc.

and around of 80 simultaneos users on these servers.

So, I will use the same LUN for data disks.

Thanks all.