Query about how well mirrored partitions protect data.

[tim.iee]

Hi Everyone.

I have a computer which has two hard drives in it (Disk 0 and Disk 1) and the the primary "C:" partition is mirrored on both hard drives using the Windows Disk Management utility.

If a platter on Disk 0 starts to degrade, causing file corruption, is there a risk that the mirroring process can copy this corrupt data over to Disk 1, thus making the original non-corrupt data unrecoverable?

Similarly, can the mirroring process copy corrupt data from Disk 1 to Disk 0? (I am guessing that the answer to this question is the same as the answer to the above question.)

Thank you very much.

Kind regards

Tim

[rogfanther]

Mostly, yes.

If they are mirrored by software ( windows ) , and assuming something would corrupt some file, windows would write the same file information it sees to both disks, thus writing the "corrupted" data .

But I think that if "a platter starts to degrade", and that causes file corruption, windows would see errors when reading from that disk, and would instead read from Disk 1. In case of no errors then, it would probably log an error in Event Log with the information about the reading errors of Disk 0.

[tim.iee]

Hi Rogfanther

Thank you very much for your reply.

If a partition is mirrored on three or more hard drives and a Bit in a file is read and all drives apart from one return a "1" and the remaining drive returns a "0" due to corruption (or the other way round), does this mean that the drive which returned a different Bit is outvoted (meaning that the majority wins)?

Thank you very much.

Kind regards

Tim

[rogfanther]

Something like that, yes.
Much will depend of the kind of mirroring used ( Hardware x Software ), but the implementations will compare/parity/checksum/etc data read and "decide" if it is correct/recoverable ( or not ) before presenting the data to the OS.

There are many differences between implementations, either hardware or software, and also a lot of reasons used to justify using one or other. Mostly, one should read about the benefits and difficulties of both in case of failures or performance, and then decide in one´s situation which is better.

[ReclaiMe]

if there is a bit error, no voting happens. The sector is requested from arbitrary drive, and if the drives returns data rather than error status, the sector data is accepted and passed up to a requesting application. Another read attempt (from a different drive) is only performed if the first attempt fails (with a read error).

On Windows at least, that is.