Question

Consider a RAID level 5 organization comprising five disks, with the parity for sets of four blocks on four disks stored on the fifth disk. How many blocks are accessed in order to perform the following?

a. A write of continuous three block of data.

b. A write of eight continuous blocks of data.

Answer 1

1. The steps involved when three contiguous blocks of data are written are as follows:

• The block which has the parity is read.
• The old data which is stored in the target block (out of the three blocks) is read.
• The new parity is computed based on the differences of the old and the new content of the target block.
• The contents of the parity block and the target block are then written.

2. The steps involved when eight contiguous blocks of data are written are as follows:

• Assume there exists a four-block boundary and these eight contiguous blocks begin at this boundary.

• These eight contiguous blocks can be written if the parity block of the first four blocks is written.
• The eighth block is read and the parity of the next set of four blocks is computed.
• The corresponding parity block is written onto the disk.