Hitachi Data Systems (HDS) introduced thin (dynamic) provisioning with its USP-V Tagmastore systems. However, some limitations have emerged about this thin provisioning implementation from an EMC blogger.

Thin provisioning is the neat trick of telling an app it has a LUN of a certain size when in fact you only actually allocate a small proportion of that disk capacity and add real disk space to it as the application's data write needs progress. The benefit is that you can avoid buying lots of disk space initially through having to allocate the maximum anticipated capacity to a LUN when you first provision the LUN.

1. HDS restricted dynamic provisioning to the USP-V (virtualised) hardware. Existing USP customers don't get it and can't upgrade to it with a software upgrade. Existing customers means existing HDS and HDS OEMs HP and Sun customers for the USP technology.

2. The USP systems can virtualise external storage. But dynamic provisioning is only available on HDS USP-V storage. That means Fibre Channel drives too; there are no SATA drives in USP-V. (Score one for EMC which is adding SATA drives to the FC ones in its new DMX-4 Symmetrix product. On second thoughts don't as DMX-4 doesn't have thin provisioning.)

HDS will add support for dynamically provisioning external storage 'soon', according to CTO Hu Yoshida. Elsewhere he writes: 'We have announced Dynamic Provisioning initially for internal storage on the USP V. As we gain experience with this technology we hope to provide it as a service to other storage systems that are externally attached.'

3. If you are replicating data then a dynamically provisioned USP-V device can't be a replica source or target either locally or remotely. That means that a USP-V array which is thinly provisioned can't have its data replicated.

4. HDS has adjusted its pricing so that physical storage allocated to a thin provisioning pool is twice the price of the same storage allocated to a normally-provisioned pool. That weakens the ROI for thin provisioning.

5. HDS management tools for dynamically-provisioned LUNs don't include the basic requirements for 'robust planning, monitoring, alerting, disaster-prevention and recovery tools.' Oops. Apparently HDS' ControlCenter lacks dynamic provisioning support.

6. HDS won't let customers de-dupe dynamically-provisioned LUNs.

7. RAID 6 and dynamic provisioning appear to be mutually exclusive.

NetApp is the only other supplier, as far as I know, to have added thin provisioning to an existing array. Such retrospective addition of a deep technology will be problematic and will take time, especially if the other parts of that supplier's technology refer to the total capacity allocation for a LUN and are not party to the LUN spoofing that thin provisioning involves.

HP has added a near-thin provisioning, Adaptive Provisioning, which does not involve LUN over-subscription.

By adding thin provisioning from the start of their product development cycle, suppliers like 3PAR have been able to enjoy a cleaner, less-fettered implementation.

EMC people may well enjoy pointing out HDS' thin provisioning limitations, but HDS might liken this to having a person who can only walk point out running limitations in another person. Let's not forget that running is better than walking and, by extension, having any thin provisioning is better than not.

However, according to the DMX-4 press release 'EMC plans to introduce thin provisioning capabilities for Symmetrix DMX in the first quarter of 2008, enabling customers to further improve storage utilization and simplify storage allocation while continuing to improve energy efficiency.'

Just to emphasise EMC's thin provisioning commitment, the new Celera has 'automated volume management with thin provisioning capabilities (which) ensures that storage capacity can be allocated quickly, with only a few mouse clicks.'

Hu Yoshida talks of thin provisioning being capacity virtualisation and existing storage virtualisation being volume virtualisation. He states: 'Over time, I expect that most storage vendors will be able to provide thin provisioning in their storage controllers. The question will be how will network-based virtualization systems like SVC and InVista provide thin Provisioning? How will they be able to combine volume virtualization with capacity virtualization?'

(SVC is IBM's SAN Volume Controller. InVista is EMC's network storage management applications that runs in a SAN director.)

That is an interesting question. Can SVC and InVista provide thin provisioning services? With EMC implementing thin provisioning on Symmetrix in calendar Q1 2008 I guess it's voted; thin provisioning is an array controller function and not a SAN director function.