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Analyzing RAID parity

Last time I discussed how to find the RAID data offset for a SNAP OS 4.x RAID handler. To put it briefly it was just a simple matter of finding Cylinder Group zero on the first drive in the array and back tracking 48 sectors. Once the RAID data offset is established we can plug those numbers into our RAID Diagnostic Toolkit and begin analyzing the parity.

The main objective of the parity check is to make sure that:

1. We do not have a stale drive in the array

2. We do not have a drive in the array that does not belong

3. All RAID data offsets are correct.

Lets take each item from one to three and explore their impact. Item one basically means that there is a drive in the array that has not been functioning for a certain period of time. Normally an alarm goes off, an email may be sent, there is some sort of notification that a drive has dropped out of the array and now the RAID is running in a degraded state. When the technician who is administering the array does not get a warning it is usually because there has been some type of hardware malfunction that, although the drive is out of the array, the RAID BIOS does not sound the alarm. A second reason is that the alarm stops working. The little speaker on the RAID card that sends this terrible shrill through the server room is malfunctioning and nobody hears it. Another reason might be that the original RAID administrator may have shut off all alarm notification flags during configuration and never turned them back on. There are a lot of other reasons but the fact of the matter is that a RAID administrator may have a RAID that has been degraded for a year and not even be aware of it.

Item two is rare, however, it happens enough to where you need to be concerned if you are trying to recover your RAID. This item also is not very common in the SNAP line of servers as it is in DELL. There are times when a RAID is configured as ‘X’ drives, and one hot swap. The RAID admin who is now working for the company you are trying to recover the data for sends the RAID he tells you it has four drives when it is really three drives and one hot swap. He may not know the original configuration. He may not know how to get into the RAID BIOS to look to see how it was configured. There could be a hundred and one reasons as to why you get a hot swap drive sent to you along with the rest of the array. The point is, be aware that it can happen.

As a side note, DELL has configure many of their RAID models to have two mirrored drives for the OS, and 3 to X drives as a RAID 5. I have received all the drives from a client with them ‘swearing’ that all of these drives are in the array. Once I have analyzed the parity, and look at the drives through a hex editor I come to the realization that I have two RAIDS on my hands, not one. Once again, be aware that the client may not know their exact configuration.

Finally item three. Sometimes, not often, actually this was the first time with a SNAP server, the RAID data offsets are staggered. In my next installment I will explain what happened with this particular job, and why it happened. Until next time.

Click here to Download the RAID Diagnostic Toolkit. Be sure to read the instructions on the page as well as follow the links to the instructions with screenshots. You may also visit our page: RAID Configuration and Parity Check for more information. Find out more about RAID Data Recovery.

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  1. SNAP Server Data Recovery - Finding SNAP OS 4.x RAID Data Offset | Hard Drive Recovery Resources From DTI Data Recovery - July 23, 2008

    […] Next step will be to check the drive parity which, in this case, was unusual. This step will be in the next blog titled “Analyzing RAID parity“. […]

  2. Hard Drive Data Recovery - Analyzing RAID parity | Data Recovery Strategies By Data Recovery Expert Jaison Jacob - July 24, 2008

    […] Analyzing RAID parity – Last time I discussed how to find the RAID data offset for a SNAP OS 4.x RAID handler. To put it briefly it was just a simple matter of finding Cylinder Group zero on the first drive in the array and back tracking 48 sectors. … […]

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