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R Bruce Hoffman wrote on Sat, 31 Oct 2009 18:24:19 GMT:

Right, but if he just adds the fourth, then the parity still
remains only on the first three... right?

Right, if you don't stop and restart RAID, the parity remains on
the original 3 drives and the 4th is added to the parity set
without any parity data.

So while SAS
improves the three disk situation, it still doesn't change the
parity implementation of raid 5... right?


Nothing really changes between SAS and SCSI as to what happens
when you add 1 or 2 drives to an existing parity set. These 1 or
2 drives will have no parity data but will be protected.

It is the basic formation of parity sets where SAS and SCSI are
different. In the SCSI world, parity could only exist on 2, 4,
8, or 16 drives while you can have anywhere from 3 to 18 drives
in a parity set. In the SAS world, parity data will be striped
across all the drives that are present when parity protection is
started up to the maximum of 18 drives in a parity set.


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