SaaS data loss: The problem you didn’t know you had

Parity spaces

Spaces can have the attribute of being parity spaces, in which case parity information about data is stored as well to aid in data-regeneration when a physical drive fails. Once again slabs are used as an intermediate storage construct and striped. Parity spaces take up less space than a mirrored copy of the data, but involve more random I/O in their operation.

When a drive fails, there is automatic recovery of the lost data, using parity we suppose, and a regeneration of the parity data, using the same general principles as with a mirror-spaces recovery operation.

You can have parity spaces and mirror spaces carved out from the same storage pool with the slabs intermingled. Parity spaces appear to be roughly equivalent to RAID 5 (single drive failure) and RAID 6 (dual drive failure protection).

Management

Storage spaces can be created using the PowerShell CLI. This is okay for storage admins, but – to this writer's mind – frankly ghastly for small businesses and home users. (Sinofsky and Nagar's blog post provides examples.)

Far better to use the Control Panel and get a GUI approach, which is simpler and cleaner. Again the blog post gives examples. You select the System and Security option, then Storage Spaces.

Storage Spaces and the control panel

So, what do we think about Storage Spaces? First of all, virtualising storage is a good idea, and automating data resilience and recovery from drive failure is very sensible. Perhaps users with Storage Spaces will have less need to rely on backup software or to buy self-protecting external storage arrays such as Drobos.

However, the protection, although RAID-like, is not RAID and not hardware-assisted. We have no information on recovery timings other than that it happens automatically in the background, which is good. Clearly, the larger the capacity of the failed drive, the longer the recovery time will be. Perhaps storage spaces are better carved out from pools made of many small drives than a few large drives.

Also, recovery uses host CPU cycles and this may, in a machine with few spare cycles, affect overall responsiveness.

A third overall point is that users will have to know when to use basic storage spaces, mirror spaces, and parity spaces. Storage user life is simpler in Drobo-land where there are fewer choices. You might feel that Microsoft is trying to cover too many bases with a Storage Spaces concept that covers all the ground and requirements between home users and enterprise data centres.

Storage Spaces is somewhat like ZFS, although it has no deduplication and lacks other ZFS features. However, it is a start – and Microsoft will probably add features such as snapshots, replication, deduplication, and, maybe, compression. El Reg also thinks that there could be a Hyper-V virtualisation angle to this – and more is to come. ®

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