Disk mirroring (RAID 1) concepts
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Data Integrity, Availability, and Protection
Disk mirroring (RAID 1) concepts
Symmetrix Mirroring (RAID 1) provides the highest level of
performance and availability for all mission-critical and
business-critical applications. Mirroring maintains a duplicate copy
of a logical volume on two physical disk drives. The Symmetrix
system maintains these copies internally by writing all modified data
to both devices. The mirroring operation is transparent to the host.
The mirroring feature designates from two to four logical volumes
residing on different physical devices as a mirrored volume, one
volume being mirror-1 and the other volumes being mirror-2,
mirror-3c, and mirror-4. The host views the mirrored volumes as the
same logical volume because each has the same unit address.
Advantages of
mirroring
Symmetrix Mirroring offers the following advantages:
â—† Improved performance over traditional mirrored (RAID 1) data
protection by supporting 100-percent fast write and two
simultaneous internal data transfer paths.
â—† Protection of mission-critical data from any single point of failure.
â—† Assurance that the second copy of data is identical to the first
copy.
â—† Superior read performance since both mirrors are used for read
operations.
â—† Continuous business operation in a situation where there is a
device failure.
â—† Automatic resynchronization of the mirrored pair after repairing
the defective volume.
Write operations
with mirroring
Symmetrix systems handle a write operation to a mirrored logical
volume as a normal write operation. The channel director presents
channel end and device end (or a good-ending status) to the channel
after data is written to, and verified in, global memory. The disk
directors then destage the data to each drive of the mirrored pair
asynchronously. As such, mirroring on Symmetrix systems exploits
the 100-percent fast write capability, and the application does not see
additional time associated with having to physically perform two
disk write I/Os (one to each drive of the mirrored pair) as normally
occurs with mirroring.
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