Common Name(s): RAID 1; RAID 1 with Duplexing.
Technique(s) Used: Mirroring or Duplexing
Description: RAID 1 is usually implemented as mirroring; a drive has
its data duplicated on two different drives using either a hardware RAID controller or
software (generally via the operating system). If either drive fails, the other continues
to function as a single drive until the failed drive is replaced. Conceptually simple,
RAID 1 is popular for those who require fault tolerance and don't need top-notch read
performance. A variant of RAID 1 is duplexing,
which duplicates the controller card as well as the drive, providing tolerance against
failures of either a drive or a controller. It is much less commonly seen than straight
mirroring.
|
Illustration of a pair of mirrored hard disks, showing
how the
files are duplicated on both drives. (The files are the same as
those in the RAID 0 illustration, except that
to save space I have
reduced the scale here so one vertical pixel represents 2 kiB.) |
Controller Requirements: Supported by all hardware controllers, both
SCSI and IDE/ATA, and also most software RAID solutions.
Hard Disk Requirements: Exactly two hard disks. Any type may be used
but they should ideally be identical.
Array Capacity: Size of Smaller Drive.
Storage Efficiency: 50% if drives of the same size are used, otherwise
(Size of Smaller Drive / (Size of Smaller Drive + Size of Larger Drive) )
Fault Tolerance: Very good; duplexing even better.
Availability: Very good. Most RAID controllers, even low-end ones,
will support hot sparing and automatic rebuilding of RAID 1 arrays.
Degradation and Rebuilding: Slight degradation of read performance;
write performance will actually improve. Rebuilding is relatively fast.
Random Read Performance: Good. Better than a single drive but worse
than many other RAID levels.
Random Write Performance: Good. Worse than a single drive, but better
than many other RAID levels. 
Sequential Read Performance: Fair; about the same as a single drive.
Sequential Write Performance: Good; again, better than many other RAID
levels.
Cost: Relatively high due to redundant drives; lowest storage
efficiency of the single RAID levels. Duplexing is still more expensive due to redundant
controllers. On the other hand, no expensive controller is required, and large
consumer-grade drives are rather inexpensive these days, making RAID 1 a viable choice for
an individual system.
Special Considerations: RAID 1 arrays are limited to the size of the
drives used in the array. Multiple RAID 1 arrays can be set up if additional storage is
required, but RAID 1+0 begins to look more attractive in that circumstance. Performance
may be reduced if implemented using software instead of a hardware controller; duplexing
may require software RAID and thus may show lower performance than mirroring.
Recommended Uses: Applications requiring high fault tolerance at a low
cost, without heavy emphasis on large amounts of storage capacity or top performance.
Especially useful in situations where the perception is that having a duplicated set of
data is more secure than using parity. For this reason, RAID 1 is popular for accounting
and other financial data. It is also commonly used for small database systems, enterprise
servers, and for individual users requiring fault tolerance with a minimum of hassle and
cost (since redundancy using parity generally requires more expensive hardware.)
Next: RAID Level 2
