MCSE : Security Specialist
Describe the main characteristics of network-attached storage.
RAID
Redundant Array of Inexpensive (or Independent) Disks. A RAID array
is a collection of drives which collectively act as a single storage
system, which can tolerate the failure of a drive without losing
data, and which can operate independently of each other.
Level
0 referred to as striping, is not redundant. Data is split across
drives, resulting in higher data throughput. Since no redundant
information is stored, performance is very good, but the failure of
any disk in the array results in all data loss.
Level
1 referred to as mirroring with 2 hard drives. It provides
redundancy by duplicating all data from one drive on another drive.
Performance is better than a single drive, but if either drive
fails, no data is lost. This is a good entry-level redundant system,
since only two drives are required.
Level
2, which uses Hamming error correction codes, is intended for use
with drives which do not have built-in error detection. All SCSI
drives support built-in error detection, so this level is not needed
if using SCSI drives.
Level
3 stripes data at a byte level across several drives, with parity
stored on one drive. It is otherwise similar to level 4. Byte-level
striping requires hardware support for efficient use.
Level
4 stripes data at a block level across several drives, with parity
stored on one drive. The parity information allows recovery from the
failure of any single drive. Performance is very good for reads.
Writes, however, require that parity data be updated each time. This
slows small random writes, in particular, though large writes or
sequential writes are fairly fast.
Level
5 striping with distributed parity. Similar to level 4, but
distributes parity among the drives. No single disk is devoted to
parity. This can speed small writes in multiprocessing systems.
Because parity data must be distributed on each drive during reads,
the performance for reads tends to be considerably lower than a
level 4 array.
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