Monday, 25 April 2011

Difference between raw data and interpretation

One should prefer to look at and to work with raw data, if at all possible.

We got an example recently when an owner complained on a hard drive because SpeedFan reported Fitness around 25%. This does not sound good. However, closer examination revealed that SpeedFan conclusion was based on an attribute with a value of 100 (raw value 0). There appeared to be no trouble at all from the raw SMART output. Upon further investigation I found that there are two versions of the firmware on the same model hard drive, one reporting a perfectly good condition as value 253 (raw 0), and the other reporting the same perfectly good condition as value 100 (raw 0). SpeedFan sees both values as originating from a single model, and decides 100 to be a failure indication.

The more complex interpretation becomes, the more suspectible it is to all sorts of quirks and glitches. Unfrotunately, the more complex interpretation requires more human effort to work from raw data, but that's another matter entirely.

Monday, 18 April 2011

Phantom drives in Disk Management

Occasionally, you can get a hard drive which is indicated Missing in Windows Disk Management. The reason for this behavior is simple: the information about the dynamic disks is replicated across all hard drives on a PC. The dynamic disks are assigned to "disk groups", and every disk in a group holds a copy of information about all the other disks in that group.

This is useful in RAID setups: should a controller or channel fail taking two or more RAID 5 members with it, the array is declared failed and cannot be mounted. Once the controller problem is resolved, the member disks are recognized back and the RAID can be brought online again.

The MBR-based (basic) disks, if removed, just disappear from the system, because these are not tracked.

Thursday, 7 April 2011

Moving RAID 1 between controllers

Is it possible to move a RAID 1 (mirror) set between controllers?

In RAID1, two disks are exact duplicates of each other as far as user data is concerned. The RAID controller metadata is probably disk-specific, but that does not practically matter.

Because the disks are identical, there is no point in moving entire RAID 1. Move one disk, then make a mirror from it. The question should thus be restated as

Is it possible to use one member disk of RAID 1 with a different controller?

The answer depends on a controller in use. With most controllers, you can connect the RAID 1 member disk in whatever way you like and the data would be accessible. There are however some controllers which append their metadata before the user data on the member disks. With these, the direct migration is not possible. Use either a same model or a compatible controller, or use something like TestDisk or DiskPatch to do a partition recovery and make the filesystem mountable again.

Sunday, 3 April 2011

Models and feedback

The most significant problem we facing now is the lack of feedback. As it was earlier discussed, the RAID recovery software works with models, not actual data. Now the difficulty is to determine how good these models are in real-world application. Relying on people feedback does not look very promising. We'll consider incorporating certain automatic dial-home system sending statistical info we can use to get a clearer view of things. The obvious alternative of buying a sample of all possible RAID controllers and NAS units for tests looks prohibitively expensive, both in terms of money and effort.