VM's per datastore

Do vmware given any specific guidance on the number of VM's per datastore, i seem to recall hearing a recommended capacity limit of 25? I this correct, or is it really specific to the capacity of the datastore? Is there any risk to storing > 25 vm's per datastore?
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We mix and match depending on need. I will say that it is a good idea to have your data stores related to function. for example ERP. With regards to total, we had over 30 machines in one huge LUN once. it worked no problem, but it feels ill advised as you have a lot of eggs in one basket.

We now have a single LUN/Data store for our ERP servers. As we use VMware SRM with Equallogic you must replicate at LUN level. While SRM does not care how many machines are there the Equallogic is having to synch changes not only from the ERP systems but from the others too.

In short we TRY to create LUNS for each "Service" - ERP, Email/IM, Infrastructure, Database's.

Hope that helps

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pma111Author Commented:
Chris - out of interest, what systems fall into infrastructure in your organisation?
Active Directory, DNS, ISA and firewalls, Print Servers

Anything that we as a department use rather than those focused on the end user.
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
There used to be a guide, but not any more.

It's now how large are your datastores, that your VMs are on!

We like to create datastores between 500-800GB
pma111Author Commented:
so saying outright you have more than 25 vms per datastore and that is inappropriate is actually potentially a false statement?
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
There is no limit, that has been published.

It depends on the datastore, and the datastore underlying technology.

No one can really, give a figure of how many VMs per datastore.

You could have two heavy VMs, which may give you datastore issues.
There is no problem with having more than 25 per datastore, but how you manage them and their growth that is important.

Lets say you create 25 100GB VM's to have 25 machines you would need 2.5TB LUN and that is not accounting for growth.

a 2.5TB LUN/Datastore is hard to manage / Snapshot /Replicate. I would back pma111's comment:- aim for datastores around 500 - 800 gig.
Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
The correct answer is IT ALL DEPENDS and here is why:

1. What storage technology are you using?  With EMC VMAX and IBM V7000, volumes in SAN are created out of multiple pools (each pool could a RAID5 or RAID6 consisting of 8 or disks and a volume may contain 5 pools).  A LUN is assigned out of a volume.  Does it make any difference if you created a 2.4 TB LUN for VMWare or 3 x 800 GB LUN
2.  Snapshots are no longer an issue as before as new SANs create the snapshot at a hardware level and can support 256 or more snapshots.  Create snapshots only when required (i.e. patch installation).  Most SANs perform a snapshot during backups and could potentially move the snapshots to a different location (i.e. a different SAN)
3.  If throughput is an issue then use faster HBAs, multiple HBAs, distribute your load across more than one SAN as well as invest in larger cache on your SAN
4.  Size your LUNs according to your hardware capabilities and business requirements.  I have seen environments where SAP data is over 1.8 TB (large SQL databases, etc.) and 500 to 800 GB would not be able to provide a single drive required for logs
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:

1. I would argue, the larger the lun, the higher the risk if it gets deleted, corrupted, the more VMs you lose!

2. These are SAN Snapshots, I think we are discussing VMware Snapshots here! Same name but different...

3. SSDs? Jumbo Frames, Multipath oh, I have an EE Article on those...

HOW TO: Enable Jumbo Frames on a VMware vSphere Hypervisor (ESXi 5.0) host server using the VMware vSphere Client

HOW TO: Add an iSCSI Software Adaptor and Create an iSCSI Multipath Network in VMware vSphere Hypervisor ESXi 4.1

HOW TO: Add an iSCSI Software Adaptor and Create an iSCSI Multipath Network in VMware vSphere Hypervisor ESXi 5.0

4. Clearly, you have to size a LUN according to size of VM! But then some VMs are not ideal candidates for virtualisation!
If you are talking VM's running that large then you should be aware of VMware vSphere maximum's. My advice is based on a smaller environment. vSphere 5.0 maximums : http://www.vmware.com/pdf/vsphere5/r50/vsphere-50-configuration-maximums.pdf

A point to note from Mnkhawaja advice. His point on hardware snap-shoting is valid but this is not default functionality. it must be configured! Equallogic for example has various integration tools to do this, but if you use standard functionality it wont use the SAN technology to snapshot.

With regards to the technology used I can only comment on Equallogic and their storage stack, unless vmware advice has changed don't spread (or extend) a data store over muliple LUN's, this is not good practice. We make a point of creating a new data store if we need more space then storage vmotion the VM's to the bigger drive.

But ultimately, Mnkhawaja is right. Without knowing your business and requirements we can only give you an idea.

hanccocka beat me to it...
As other Experts have mentioned there is not hard set rule stating you can only run X number of VM's per datastore. What is comes down to is how much IOPS each VM will produce, how much your SAN can handle.

For example, we have several LUNS / volumes which host 30 - 50 VM which are low IOPS producing but then we have a hand full of LUNS / volume which one host one or two very high IOPS generating VM's.

You never want to mix high IOPS vm's with lower IOPS VM's because the high IOPS VM's may drag down performance on the other VM's..

hope this makes sense,,,
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