DAS-208SA Disk Array User Guide
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(grouped) data streams (like RAID 0) and a single parity disk records the
generated parity for the data blocks. A data file that is to be recorded is broken
up into blocks and recorded on the data disks, with its corresponding parity
block(s) recorded on the data disks. Because all disks together transfer each
data file, the number of data disks multiplies the data transfer rate, but the
transaction rate is that of a single disk. Both read and write operations involve
all array disks.
If a disk fails, the data from the good disks can recreate the failed data via
generating parity. Which disk failed is known from the fact that a disk is not
responding or has reported an ECC error.
A.6 RAID 4
Rather than being broken up into blocks and stored on multiple data disks, as a
RAID 3 data file is, RAID 4 stores the entire data file on a single disk. The
corresponding location on the parity disk contains the parity for not just the first
data file, but also for the other (unrelated) data files stored on the
corresponding locations on the other data disks.
Data transfer rates are the same
per data file
as single disk transfer rates, but
multiple (as many as there are data disks) files may be simultaneously read.
This generates a high read transaction rate and good aggregate read transfer
rates. However, RAID 4 is seldom implemented because of the problems that
ensue in the event of a read error or any write operation.
A RAID 4 write operation requires two disk revolutions for the following
sequence:
