< RAID
Levels Explained
by Broadberry
What
is RAID?
The basic idea of RAID is to combine multiple small, inexpensive disk drives into an array of disk
drives which yields performance exceeding that of a Single Large Expensive Drive (SLED). Additionally, this array of drives appears to
the computer as a single logical storage unit or drive.
The
Mean Time Between
Failure (MTBF) of
the array will be
equal to the MTBF
of an individual
drive, divided by
the number of drives
in the array. Because
of this, the MTBF
of an array of drives
would be too low
for many application
requirements. However,
disk arrays can be
made fault-tolerant
by redundantly storing
information in various
ways. Basics:
- RAID
0 is
the fastest
and most
efficient
array type
but offers
no fault-tolerance.
- RAID
1 is
the array
of choice
for performance-critical,
fault-tolerant
environments.
In addition,
RAID-1 is
the only
choice for
fault-tolerance
if no more
than two
drives are
desired.
- RAID
2 is
seldom used
today since
ECC is embedded
in almost
all modern
disk drives.
- RAID
3 can
be used in
data intensive
or single-user
environments
which access
long sequential
records to
speed up
data transfer.
However,
RAID-3 does
not allow
multiple
I/O operations
to be overlapped
and requires
synchronized-spindle
drives in
order to
avoid performance
degradation
with short
records.
- RAID
4 offers
no advantages
over RAID-5
and does
not support
multiple
simultaneous
write operations.
- RAID
5 is
the best
choice in
multi-user
environments
which are
not write
performance
sensitive.
However,
at least
three, and
more typically
five drives
are required
for RAID-5
arrays.
- RAID
6 is
similar to
RAID level
5 however
it allows
extra fault
tolerance
by using
a second
indipendent
parity scheme.
- RAID
10 is
implemented
as a striped
array whose
segments
are RAID
1 arrays.
- RAID
0+1 is
a mirrored
array whose
drives are
in a RAID
5 array.
RAID
Level 1 RAID
Level 1 provides
redundancy by
writing all
data to two
or more drives. The
performance
of a level 1
array tends
to be faster
on reads and
slower on
writes compared
to a single drive,
because the
data is split
between numerous
hard-drives,
but if
either drive
fails, no
data is lost.
This is a good entry-level
redundant set-up,
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. This
level is commonly
referred to
as mirroring.
 > Go
to RAID 2
|
