RAID is an acronym for 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.
What are the different RAID levels?
A research group at UC-Berkeley coined the term "RAID", defining six
RAID levels. Each level is a different way to spread data across
multiple drives--a compromise between cost and speed. Understanding
these levels is important, because each level is optimized for a
different use.
RAID Level 0
RAID Level 0 is not redundant, hence does not truly fit the "RAID"
acronym. In Level 0, 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. This level is commonly referred to as
striping.
RAID Level 1
RAID Level 1 is commonly referred to as mirroring with 2 hard drives.
It provides redundancy by duplicating all data from one drive on
another drive. The performance of a Level 1 array is slightly 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. However, since one drive is used to store a duplicate of
the data, the cost per megabyte is high.
RAID Level 2
RAID 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 of
little use when using SCSI drives.
RAID Level 3
RAID 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.
RAID Level 4
RAID 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. The performance of a
level 4 array is very good for reads (the same as level 0). 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. Because only one drive in the array stores
redundant data, the cost per megabyte of a level 4 array can be
fairly low.
RAID Level 5
This level is commonly referred to as striping with distributed
parity. RAID Level 5 is 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. The cost per
megabyte is the same as for level 4.
RAID 0/1 or 10
RAID 0/1 is a dual level array that utilizes multiple RAID1
(mirrored) sets into a single array. Data is striped across all
mirrored sets. As a comparison to RAID 5 where lower cost and fault
tolerance is important, RAID 0/1 utilizes several drives, in order to
provide better performance. Each drive in the array is duplicated
(mirrored). This eliminates the overhead and delay of parity. This
level array offers high data transfer advantages of striped arrays
and increased data accessibility (reads). System performance during a
drive rebuild is also better than that of parity based arrays, since
data does not need to be regenerated from parity information, but
copied from the other mirrored drive.
RAID 0/5 or 50
RAID 0/5 is a dual level array that utilizes multiple RAID5 sets into
a single array. In RAID 0/5 array, a single hard drive failure can
occur in each of the RAID5 without any loss of data on the entire
array. Keep in mind, as the number of hard drives increase in an
array, so too, does the increased possibility of a single hard drive
failure. Although there is an increased write performance in RAID
0/5, once a hard drive fails and reconstruction takes place, there is
a noticeable decrease in performance, data/program access will be
slower, and transfer speeds on the array will be effected.
Which RAID level should I use?
The right choice depends on the application. The RAID Levels below
provide a brief summary and general uses. Keep in mind, Adaptec RAID
controllers do not support all RAID Levels. Please check Technical
Specifications for your RAID product for RAID Levels that are
supported.
RAID Level Uses
Level 0 (striping)
Any application which requires very high speed storage, but does not
need redundancy. Photoshop temporary files are a good example.
Level 1 (mirroring)
Applications which require redundancy with fast random writes; entry-
level systems where only two drives are available. Small file servers
are an example.
Level 0/1 or 10 (mirroring and striping)
Dual level raid, combines multiple mirrored drives (RAID 1) with data
striping (RAID 0) into a single array. Provides highest performance
with data protection.
Level 5 (distributed parity)
Similar to level 4, but may provide higher performance if most I/O is
random and in small chunks. Database servers are an example.
Level 0/5 or 50
Dual level raid, combines multiple RAID 5 sets with data striping
(RAID 0). Increased reliability and performance over standard RAID5
that can stand a multiple drive failure; one hard drive per RAID5
set.
Copyright 1999 Adaptec, Inc. All rights reserved. Adaptec, the
Adaptec logo, and the Adaptec tagline are trademarks of Adaptec, Inc.
which may be registered in some jurisdictions. Windows NT is a
registered trademark of Microsoft Corporation, used under license.
All other trademarks are owned by their respective owners.
---
* Origin: [adminz] tech, security, support (192.168.0.2)
generated by msg2page 0.06 on Jul 21, 2006 at 19:04:06
search: