Data Recovery Weekly

When recovering data from a client's NETGEAR ReadyNAS device we saw a rather strange layout - RAID5 of two disks, which appeared once ReclaiMe File Recovery has processed md-raid records. After looking at it for a while we realized that this is, surprisingly, possible given that we ignore the requirement of a minimum number of disks. So let's see what happens when placing data and parity blocks on two disks in a RAID5 layout where a stripe contains, say, three sectors: Now let's see what are there in the parity blocks in such a layout? In order to get an even row of two elements it is needed that all the elements would be the same, so the content of the parity block is the same as the content of its corresponding data block. Therefore we deal with a typical RAID1 (mirror) layout. Despite all the wildness of the layout, it meets all the basic criteria of a RAID5: survives a single disk failure, the disk space overhead equals to the capacity of one member disk....

I'm getting tired of people advocating RAID 5 vs. RAID 6. They go on like oh, in a RAID 5 a bit error URE will get you one day! We spent 10,000s dollars sending our RAID5s to OnTrack. Yes, that's tens of thousands of Uncle Sam Dollars. Single bit error in a RAID 5 is much cheaper to recover from than a RAID 6 controller failure or an operator accidentally deleting the array. Ever asked for a quote on RAID 6 recovery?

I recall we discussed that earlier, that our ReclaiMe Free RAID recovery software (and pretty much any generic RAID recovery software, like Runtime's RAID Reconstructor) does not actually work with RAIDs. It works with models of RAIDs instead. Data recovery service view on RAID5 more like "disks (by specific vendor), controller (also by specific vendor), and cables". The software sees it like 1 2 P 3 P 4 P 5 6 and pretty much nothing else. The problem arises when the actual data does not match the model used in software. The synchronous array (below) cannot be described in terms of the asynchronous model (above). 1 2 P 4 P 3 P 5 6 So the software has to acount for all possible models of the RAID 5, of which there are quite a number. We're currently working on automatic analysis of the so-called delayed parity arrays, used in HP SmartArray controllers. This subtype of RAID5 does effectively have two distinct stripe sizes, one for data and the other (...