Re: RAID LICENSE - any one using and having fun with it?

R.A.Omond wrote:
Be aware that there is potential (though highly unlikely) to lose
data if you're using RAID-5 and you have a system crash at an
inopportune moment, since there's effectively no "battery-backed
cache" as you would normally have in any other RAID-5 controller.

No, this software implementation of RAID-5 very carefully protected against the so-called "write hole", where either data or the associated parity (but not both) have been updated at the point where a system crashes.

The software used reserved areas scattered across the disk surface to store metadata, and used this as its non-volatile storage in lieu of the non-volatile cache memory available to a controller pair.

But one consequence of this "fail-safe" design was that writing to disk was then even slower with this host-based product, because it would first set the metadata bits on disk to indicate the parity associated with a stripe of data was being modified, then modify the data and parity, then re-set the metadata bits to indicate that the data and parity were now consistent.

(In practice the overhead of these additional writes can be mitigated by doing putting the disks onto a controller with write-back cache so the controller hides the latency of the additional writes, and they don't actually have to land on disk. But if you have such a controller it usually has RAID-5 built in, so you might as well use that, unless you are using the host-based software to do RAID-5 across multiple controllers to protect against the failure of a controller.)
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