Saturday, May 21, 2011

Hybrid RAID

Digital photography needs a lot of storage.
A typical RAW file is about 1MB per megapixel, or more, so 100 pictures taken with a 16 megapixel camera may need 1.6 GB, which is usually to be multiplied by a constant factor because most programs will cache on the disk every kind of preview.
Adobe Lightroom needs 1 or 2 GB for CameraRaw output, plus it stores all the previews (standard and 1:1).
So it makes sense to have a very large drive for the pictures, and a very fast drive for the cache, but I'd really like to combine them in a single device.

There are some controllers that combine one small SSD drive and one large traditional drive, where the former is just a cache for the second, for example Silverstone HDDBOOST
With Silverstone card, the final capacity of the device is that of the magnetic drive, and the SSD transparently vanishes.

I was hoping that Adaptec Hybrid RAID solution would be a more complete solution, but it is not.
A hybrid RAID is simply a RAID 1 where one of the disks is a SSD; all reads are redirected to the SSD, while writes are replicated on both disks.
But the final size of a hybrid volume is the smaller of the sizes of SSD and the other drive, so it's just standard redundancy.

What is needed is actually Adaptec MaxIQ; this combines any number of traditional drives in whatever RAID and one or more SSDs, but it is also seriously expensive; you need either
  1. a special Q series Adaptec controller and a certified SSD (typically an enterprise-grade, so same performance as a corresponding consumer model, but not exactly cheap)
  2. a standard 2xxx or 5xxx Adaptec controller and Adaptec Max Cache, which is just a modified 32GB Intel X25 (now obsolete), sold at a monster price.

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