Jim Kelly (The Storage Buddhist blog) has an interesting post talking about using IBM’s San Volume Controller (SVC) in front of their new FlashSystem arrays to create a scale-out all-flash solution. The overhead is a mere 100 microseconds, which as Jim indicates still makes a hybrid SVC/FlashSystem faster than many array solutions on the market. It also adds extra functionality via the SVC. However is scale-out capacity the only consideration with this design?
SAN Volume Controller
SVC is IBM’s storage virtualisation appliance, which takes traditional Fibre Channel storage arrays and abstracts their storage behind the SVC servers (or nodes), creating virtual LUNs which appear to the host to be part of the SVC itself. Storage virtualisation is not new, with hardware solutions from Hitachi (VSP), HP (XP24000), EMC (VPLEX & VMAX) and NetApp (clustered ONTAP), some of which have been on the market for 10 years. There are also a number of software only solutions available too, which convert standard Windows/Linux servers into virtualisation appliances. The benefits are focused around operability; storage virtualisation enables physical resources to be replaced and reconfigured dynamically, enables transparent migrations (once the appliance is in place) and allows the re-use of older storage resources. It’s even possible to accelerate cheaper storage using SVC with flash and their EasyTier feature, for example.
So while we can easily extol the virtues of storage virtualisation, what are the disadvantages? Well, there are a few:
- Complexity – solutions can be more complex, with data existing in multiple levels of cache, making fault determination more difficult.
- Support – support for the underlying storage has to be provided by the virtualisation vendor, which needs validation with each code upgrade (however this also simplifies the front-end connection support matrix).
- Manageability – it’s easy for virtualisation solutions to get messy without adequate standards and that can also lead to performance problems.
- Maturity – a virtualisation layer requires some key functionality (such as replication) to move up to the virtual layer. Advanced features (tiering, replication, etc) are not supported across all solutions.
Ultimately when deciding to use storage virtualisation, it’s all about comparing the benefits against the disadvantages. But what about SVC? Jim’s article was specifically talking about scale-out and this where SVC has a particular problem.
Mdisks and Vdisks
SVC uses the concepts of extents to map physical storage (Mdisks) to virtual volumes (Vdisks). Physical storage is divided into extents, deployed in pools and then recombined from the pools to create virtual volumes. The extent is the feature that enables wide striping, thin provisioning, data migrations and tiering. However, SVC is very limited in the number of extents it can support – only 2^22 or 4,194,304 – just over 4 million. SVC copes with the issues of scale by simply deploying larger extents. For example, with an extent size of 16MB, total capacity is limited to 64TB. To support, say four FlashSystem 840 devices with a total capacity of 192TB, extents of 48MB or larger are needed, which means in practice using 64MB as this is the next available permitted size. To achieve maximum LUN (volume) scalability of 8192 volumes would need an 8-node SVC cluster. Each volume would need to be a minimum of 24GB in size simply to use the available capacity.
Now, these limits may not seem a problem, but remember an extent can only be assigned to one volume. This means for space efficient (thin provisioned) volumes, the growth unit would be 64MB, a heavily wasteful size as host file system fragmentation occurs, requiring lots of host-based re-organisation to keep tidy. So for our scale out solution we have an 8-node SVC to support four storage nodes using 64MB blocks – a totally imbalanced design. One final thought on this; if the solution was required to scale further, then the extent size would have needed to be set at deployment time, meaning scale-out requirements would have to have been included in that initial planning and potentially making thin provisioning even more wasteful.
The Architect’s View
I’m a big fan of storage virtualisation and SVC has many great features. However scale-out is not one of them. The system may be capable of scaling capacity, but the poor number of logical volumes and extents it supports is severely limiting to many deployments. I am surprised that this limit has been in place for many version releases of the software; it really needs an upgrade. Pairing FlashSystem with SVC is a great idea, but the restrictions around configuration (and of course the additional cost) make the solution impractical compared to more mature all-flash solutions already on the market. While SVC & FlashSystem may work as a technical solution, the practicalities and cost make no sense at all.
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