RAID
The basic idea of RAID (Redundant Array of Independent Disks) is to
combine multiple inexpensive disk drives into an array of disk drives to
obtain performance, capacity and reliability that exceeds that of a single
large drive. The array of drives appears to the host computer as a single
logical drive.
The Mean Time Between Failure (MTBF) of the array is equal to the
MTBF of an individual drive, divided by the number of drives in the array.
Because of this, the MTBF of a non-redundant array (RAID 0) is too low
for mission-critical systems. However, disk arrays can be made fault-
tolerant by redundantly storing information in various ways.
Five types of array architectures, RAID 1 through RAID 5, were originally
defined, each provides disk fault-tolerance with different compromises in
features and performance. In addition to these five redundant array
architectures, it has become popular to refer to a non-redundant array of
disk drives as a RAID 0 array.
Disk Striping
Fundamental to RAID technology is striping. This is a method of
combining multiple drives into one logical storage unit. Striping partitions
the storage space of each drive into stripes, which can be as small as one
sector (512 bytes) or as large as several megabytes. These stripes are then
interleaved in a rotating sequence, so that the combined space is
composed alternately of stripes from each drive. The specific type of
operating environment determines whether large or small stripes should
be used.
Most operating systems today support concurrent disk I/O operations
across multiple drives. However, in order to maximize throughput for the
disk subsystem, the I/O load must be balanced across all the drives so that
each drive can be kept busy as much as possible. In a multiple drive
system without striping, the disk I/O load is never perfectly balanced.
Some drives will contain data files that are frequently accessed and some
drives will rarely be accessed.
STRIPING DISK DRIVES
Disk 1 Disk 2 Disk 3 Disk 4
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
Data stripes from each drive
are interleaved to create one logical drive.
By striping the drives in the array with stripes large enough so that each
record falls entirely within one stripe, most records can be evenly
distributed across all drives. This keeps all drives in the array busy during
heavy load situations. This situation allows all drives to work concurrently
on different I/O operations, and thus maximize the number of
simultaneous I/O operations that can be performed by the array.
Definition of RAID Levels
RAID 0 is typically defined as a group of striped disk drives without parity
or data redundancy. RAID 0 arrays can be configured with large stripes for
multi-user environments or small stripes for single-user systems that
access long sequential records. RAID 0 arrays deliver the best data
storage efficiency and performance of any array type. The disadvantage is
that if one drive in a RAID 0 array fails, the entire array fails.
RAID 0
Non-Redundant Striped Array
Writes can occur simultaneously on every drive.
DATA DATA DATA DATA DATA DATA
Reads can occur simultaneously on every drive.
RAID 1, also known as disk mirroring, is simply a pair of disk drives that
store duplicate data but appear to the computer as a single drive. Although
striping is not used within a single mirrored drive pair, multiple RAID 1
arrays can be striped together to create a single large array consisting of
pairs of mirrored drives. All writes must go to both drives of a mirrored
pair so that the information on the drives is kept identical. However, each
individual drive can perform simultaneous, independent read operations.
Mirroring thus doubles the read performance of a single non-mirrored
drive and while the write performance is unchanged. RAID 1 delivers the
best performance of any redundant array type. In addition, there is less
performance degradation during drive failure than in RAID 5 arrays.
RAID 1
Mirrored Arrays
Duplicate data is written to pairs of drives.
DATA DATA DATA DATA DATA DATA
Reads can occur simultaneously on every drive.
RAID 2 arrays sector-stripe data across groups of drives, with some
drives assigned to store ECC information. Because all disk drives today
embed ECC information within each sector, RAID 2 offers no significant
advantages over other RAID architectures and is not supported by
Adaptec RAID controllers.
RAID 2
Parallel Array with ECC
Each write operation spans all drives.
DATA DATA DATA DATA ECC ECC ECC
Each read operation spans all drives.
RAID 3, as with RAID 2, sector-stripes data across groups of drives, but
one drive in the group is dedicated to storing parity information. RAID 3
relies on the embedded ECC in each sector for error detection. In the case
of drive failure, data recovery is accomplished by calculating the exclusive
OR (XOR) of the information recorded on the remaining drives. Records
typically span all drives, which optimizes the disk transfer rate. Because
each I/O request accesses every drive in the array, RAID 3 arrays can
satisfy only one I/O request at a time. RAID 3 delivers the best
performance for single-user, single-tasking environments with long
records. Synchronized-spindle drives are required for RAID 3 arrays in
order to avoid performance degradation with short records. Because
RAID 5 arrays with small stripes can yield similar performance to RAID 3
arrays, RAID 3 is not supported by Adaptec RAID controllers.
RAID 3
Parallel Array with Parity
Read and write operations span all drives.
DATA DATA DATA DATA DATA PARITY
Parallel access decreases data transfer time
for long sequential records.
RAID 4 is identical to RAID 3 except that large stripes are used, so that
records can be read from any individual drive in the array (except the
parity drive). This allows read operations to be overlapped. However, since
all write operations must update the parity drive, they cannot be
overlapped. This architecture offers no significant advantages over other
RAID levels and is not supported by Adaptec RAID controllers.
RAID 4
Parallel Array with Parity
Every write must update dedicated parity drive.
DATA DATA DATA DATA PARITY
Reads can occur simultaneously
on every data drive.
RAID 5, sometimes called a Rotating Parity Array, avoids the write
bottleneck caused by the single dedicated parity drive of RAID 4. Under
RAID 5 parity information is distributed across all the drives. Since there
is no dedicated parity drive, all drives contain data and read operations
can be overlapped on every drive in the array. Write operations will
typically access one data drive and one parity drive. However, because
different records store their parity on different drives, write operations
can usually be overlapped.
RAID 5
Striped Array with Rotating Parity
Writes require parity to be updated.
DATA/ DATA/ DATA/ DATA/ DATA/ DATA/
PARITY PARITY PARITY PARITY PARITY PARITY
Reads can occur simultaneously on every drive.
In summary:
• RAID 0 is the fastest and most efficient array type but offers no fault-
tolerance. RAID 0 requires a minimum of two drives.
• RAID 1 is the best choice for performance-critical, fault-tolerant
environments. RAID 1 is the only choice for fault-tolerance if no more
than two drives are used.
• RAID 2 is seldom used today since ECC is embedded in all hard drives.
RAID 2 is not supported by Adaptec RAID controllers.
• RAID 3 can be used to speed up data transfer and provide fault-
tolerance in single-user environments that access long sequential
records. However, RAID 3 does not allow overlapping of multiple I/O
operations and requires synchronized-spindle drives to avoid
performance degradation with short records. Because RAID 5 with a
small stripe size offers similar performance, RAID 3 is not supported
by Adaptec RAID controllers.
• RAID 4 offers no advantages over RAID 5 and does not support
multiple simultaneous write operations. RAID 4 is not supported by
Adaptec RAID controllers.
• RAID 5 combines efficient, fault-tolerant data storage with good
performance characteristics. However, write performance and
performance during drive failure is slower than with RAID 1. Rebuild
operations also require more time than with RAID 1 because parity
information is also reconstructed. At least three drives are required
for RAID 5 arrays.
Dual-Level RAID
In addition to the standard RAID levels, Adaptec RAID controllers can
combine multiple hardware RAID arrays into a single array group or
parity group. In a dual-level RAID configuration, the controller firmware
stripes two or more hardware arrays into a single array.
NOTE The arrays being combined must both use the same RAID level.
Dual-level RAID achieves a balance between the increased data
availability inherent in RAID 1 and RAID 5 and the increased read
performance inherent in disk striping (RAID 0). These arrays are
sometimes referred to as RAID 0+1 or RAID 10 and RAID 0+5 or
RAID 50.
Creating Data Redundancy
RAID 5 offers improved storage efficiency over RAID 1 because only the
parity information is stored, rather than a complete redundant copy of
all data. The result is that three or more drives can be combined into a
RAID 5 array, with the storage capacity of only one drive dedicated to
store the parity information. Therefore, RAID 5 arrays provide greater
storage efficiency than RAID 1 arrays. However, this efficiency must be
balanced against a corresponding loss in performance.
The parity data for each stripe of a RAID 5 array is the XOR of all the data
in that stripe, across all the drives in the array. When the data in a stripe is
changed, the parity information is also updated. There are two ways to
accomplish this:
The first method is based on accessing all of the data in the modified
stripe and regenerating parity from that data. For a write that changes
all the data in a stripe, parity can be generated without having to read
from the disk, because the data for the entire stripe will be in the
cache. This is known as full-stripe write. If only some of the data in a
stripe is to change, the missing data (the data the host does not write)
must be read from the disks to create the new parity. This is known as
partial-stripe write. The efficiency of this method for a particular
write operation depends on the number of drives in the RAID 5 array
and what portion of the complete stripe is written.
The second method of updating parity is to determine which data bits
were changed by the write operation and then change only the
corresponding parity bits. This is done by first reading the old data
which is to be overwritten. This data is then XORed with the new data
that is to be written. The result is a bit mask which has a 1 in the
position of every bit which has changed. This bit mask is then XORed
with the old parity information from the array. This results in the
corresponding bits being changed in the parity information. The new
updated parity is then written back to the array. This results in two
reads, two writes and two XOR operations. This is known as read-
modify-write.
RAID 5
Updating Parity During Writes
XOR XOR
WRITE READ READ WRITE
DATA PARITY
The cost of storing parity, rather than redundant data as in RAID 1, is the
extra time required for the write operations to regenerate the parity
information. This additional time results in slower write performance for
RAID 5 arrays over RAID 1. Because Adaptec RAID controllers generate
XOR in hardware, the negative effect of parity generation is primarily from
the additional disk I/O required to read the missing information and write
the new parity. Adaptec RAID controllers can generate parity using either
the full- or partial-stripe write algorithm or the read-modify-write
algorithm. The parity updated method chosen for any given write
operation is determined by calculating the number of I/O operations
needed for each type and choosing the one with the smallest result. To
increase the number of full stripe writes, the cache is used to combine
small write operations into larger blocks of data.
Handling I/O Errors
Adaptec RAID controllers maintain two lists for each RAID 5 array: a Bad
Parity List, and a Bad Data List. These lists contain the physical block
number of any parity or data block that could not be successfully written
during normal write, rebuild or dynamic array expansion operations.
These lists alert the controller that the data or parity in these blocks is not
valid. If the controller subsequently needs data from a listed block and
cannot recreate the data from existing redundant data, it returns an error
condition to the host.
Blocks are removed from the Bad Parity List or the Bad Data List if the
controller successfully writes to them on a subsequent attempt.
Degraded Mode
When a drive fails in a RAID 0 array, the entire array fails. In a RAID 1
array, a failed drive reduces read performance by 50%, as data can only be
read from the remaining drive. Write performance is increased slightly
because only one drive is accessed. A RAID array operating with a failed
drive is said to be in degraded mode.
RAID 5 arrays synthesize the requested data by reading and XORing the
corresponding data stripes from the remaining drives in the array. For
RAID 5, the magnitude of the performance impact in degraded mode
depends on the number of drives in the array. An array with a large
number of drives will experience more performance degradation than an
array with small number of drives.
Rebuilding a Failed Hard Drive
A failed drive can be replaced in a RAID 1 or RAID 5 array by physically
removing the drive and replacing it or by a designated Hot Spare.
Adaptec RAID controllers will rebuild the data for the failed drive onto the
new drive or Hot Spare. This rebuild operation occurs online while normal
host reads and writes are being processed by the array.
RAID 1 arrays are rebuilt relatively quickly, because the data is simply
copied from the duplicate (mirrored) drive to the replacement drive. For
RAID 5 arrays, the data for the replacement drive must be synthesized by
reading and XORing the corresponding stripes from the remaining drives
in the array. RAID 5 arrays that contain a large number of drives will
require more time for a rebuild than a small array.
