RAID Data
Recovery
Exchange Gurus provides recovery services on
complex RAID, NAS and SAN systems. Call now 1-718-487-9764
to speak to one of our engineers about your RAID
Data Recovery issues.
RAID - Redundant Array of Inexpensive
(or Independent) Disks. A RAID 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 operate independently
of each other.
RAID Level 0 - is not redundant
and does not exactly fit the "RAID"
acronym. In Level 0, data is split across drives.
No redundant information is stored and performance
is good, however the failure of any disk in the
array results in all data loss. This level is
referred to as striping.
RAID Level 1 - 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 better
than a single drive, but if either drive fails,
no data is lost. Only two drives are required
which makes this a good entry-level redundant
system. However, the cost per megabyte is high
because only one drive is used to store a duplicate
of the data.
RAID Level 2 - uses Hamming
error correction codes. It 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 - 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 the most efficient use.
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). However, writes
require that parity data be updated each time.
This slows small random writes, in particular,
though large writes or sequential writes are fairly
fast. The cost per megabyte of a level 4 array
can be low because only one drive in the array
stores redundant data.
RAID Level 5 - is most often
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 or10 - 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 - 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 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.
