General :: ReadyNAS Duo - Under What Circumstances Resync Really Needed?

On the ReadyNAS Duo, a RAID resync synchronizes the entire contents of one disk to another. It's a slow operation. So it's not something to be done lightly. By default, it is done whenever the unit is shut down uncleanly (generally the result of a hang or power interruption). It's something that can be disabled, though, such that a resync would be done only when manually requested. The authors of the software RAID driver in Linux (called "md") came up with a nice solution to the problem of slow RAID resyncs: the md driver allows you to assign a write intent bitmap to an md device, and where it's stored is configurable (the default is to store it on the md device in question, in the superblock). So when the array goes down unexpectedly (e.g., in a power outage), only the blocks that the md device was going to write will actually be resynced.

It's sort of like a journal for the md device itself. Now, the ReadyNAS Duo doesn't seem to use Linux md at all for implementing the RAID volume, but it does have an option to disable automatic resync in the event of an improper shutdown. And that leads to my question: how necessary is it to do a resync in such an event? If the Duo uses the same method as the Linux md driver, then obviously such a resync would rarely be necessary. But if it doesn't, then it would be the equivalent of using the md driver without a write-intent bitmap, and a resync would almost certainly be necessary in the event of an improper shutdown. Does the Duo (and maybe other units) use the equivalent of a write intent bitmap for RAID write operations?