Apple XSERVE – Non-standard RAID recovery.
A recent CDR RAID-5 case study showed an example of a typical RAID-5 data recovery. This was considered to be ‘typical’ on the basis that there was multiple disk failure, and that the RAID controller was distributing data across the disks in one of the common configurations.
Generally, we considered the following configurations to be standard, which can be readily be emulated by data recovery software:
- Left-Symmetric (backward dynamic)
- Left-Asymmetric (backward)
- Right-Symmetric (forward dynamic)
- Right-Asymmetric (forward)
A typical block/stripe size is usually between 16 to 1024 sectors.
However, CDR does receive RAID arrays which do not conform to these configurations. This might include unusual stripes sizes, delays in parity, or uncommon distribution of parity.
Distribution of Parity
Many RAID levels employ protection for the data by use of a scheme named “parity”. This allows some fault tolerance. In a typical RAID 5 configuration, it allows for a failure of a single hard disk drive without the array to suffer downtime or data loss.
The parity data can be distributed in a number of different ways. Apple XSERVE RAID controllers can employ a non-standard parity block map, but it is dependent on the number of disks in the XSERVE and the model of server.
CDR has received in XSERVE machines with 3-disks. These tend to use a fairly standard forward parity based configuration with a large stripe size. Matters become more interesting with Apple XSERVE machines which have 4 or more disks. This sort of RAID recovery requires the ability to create custom block maps, as there is currently no software which can directly emulate the XSERVE RAID parity configuration.
As you might be able to see from the parity pattern below, it actually looks like an ‘X’ shape.
|Disk A||Disk B||Disk C||Disk D|
Proffesional RAID Data Recovery
CDR – Manchester Data Recovery Services – provides professional recovery services for all RAID types. All work is only payable on successful recovery, and we can offer a fixed price quotation over the phone in advance of receiving your RAID array. Please contact us if you require assistance in the recovery of your organisation’s critical data.
CDR is also happy to examine RAID arrays which have been declared ‘unrecoverable’ by other companies, frequently a full recovery will be made.