Don’t partition your ESX LUNs!

Under some circumstances you may feel the need to partition an ESX LUN, so you can create two or more datastores. Why? Let’s say you have a small branch office with a single ESX host and it’s only using internal storage from a RAID controller. And for some reason you need two datastores. Maybe it’s to meet security requirements, or data separation requirements. Maybe you can’t store your VMs and templates on the same datastore because it’s against security policy.

Given these circumstances, you may be attempted to use traditional disk partitions and then format each with VMFS to get your two or more datastores. Bzzzt! Under no circumstances does VMware support a single LUN which is partitioned and has two or more datastores. Why? It’s pretty simple actually. ESX utilizes SCSI reservations to lock a LUN and update disk metadata. During this lock, VMs can’t write to the disk. If you have two or more datastores on a single LUN, the reservations can step on each other and create problems. Expect problems!

Don’t try and be creative and pre-partition a LUN prior to installing ESX. Also, don’t try to add a new datastore and not allocate all of the capacity in the hopes you can add another datastore in the free space. Impossible.

Solutions? Well, if your server has multiple drives then configure independent RAID arrays. For example, if your server has four drives, create two RAID-1 mirrors. This gives you two LUNs, and two datastores. Alternatively, use external shared disk storage which supports multiple datastores like NFS or iSCSI.

If you have two hosts, then you could use something like the HP LeftHand iSCSI virtual storage appliance which mirrors storage between the two nodes and then present multiple iSCSI datastores to each ESX host. This could have other uses as well, such as disaster recovery, as the Lefthand VSA supports VMware Site Recovery Manager (SRM) and remote data replication.

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