Accusys RAID Controller ACS-77100 - Manual (Page 9)

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User’s Manual
1.3.3. RAID 5 (striping with parity) and RAID 5+hot spare
RAID 5 uses a mathematical expression that compares data from
three drives and calculates a fourth piece of data called “parity”
which is saved on a fourth drive. Should one of the drives failed,
parity data can be used to rebuild the failed data. Under RAID 5,
parity data is stored across all drives in the array. This maximizes
the amount of storage capacity available from all drives in the
array while still providing data redundancy. RAID 5 requires at
least three drives. The ACS-77100 allows user to set RAID 5 with
three drives and the fourth drive as a hot-spare drive ready to be
used for rebuilding data in case one of the other drives fails.
The diagram above represents the writing of data on a RAID 5 array composed of four disks
connected to the controller. Parity blocks are represented by the letter P
Characteristics
ƒ Each entire data block is written on a data disk. Parity for blocks in the same rank is
generated on Writes, recorded in a distributed location and checked on Reads.
ƒ Highest Read data transaction, medium Write data transaction rate.
ƒ Relatively low ratio of ECC (Parity) disks to data disks means high efficiency (compared to
other RAID levels).
ƒ Good aggregate transfer rate.
Recommended Uses
ƒ File and application servers.
ƒ Database servers.
ƒ Web, Email servers.
ƒ Internet server