< 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
of RAID Levels:
- 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 5 RAID
Level 5 is
similar to
level 4, but
distributes
parity among
the drives.
This can speed
small writes
in multiprocessing
systems, since
the parity
disk does not
become a bottleneck.
Because parity
data must be
skipped on
each drive
during reads,
however, 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.
 > Go
to RAID 6
|
