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RAID-1
RAID-1 is also known as mirroring or dual copy. It provides
redundancy with better performance than a single disk, but requires
real disk capacity of two times the data size.
A RAID-1 array has two disk drive members. If the members have
different capacities, the resulting RAID-1 array has the capacity of
the smaller member. Each data block of the array is written on each
of the two members.
If one member fails, operations continue, to the good member.
Performance is degraded for read operations (because read
operations cannot now be shared between the two members), and
improved for write operations (because only a single member is
written). So a mixed workload may not show any change in overall
performance. When a member returns or it is replaced (either
manually or automatically by the hot-spare mechanism), a rebuild
occurs. This rebuild is performed in parallel with any activity
to the array.
Hot Spares
A hot-spare disk drive is a disk drive that is defined for automatic use
if a disk drive within an array fails. The hot-spare must have a
storage capacity greater than or equal to that of the smallest member
of an array. You can define as many hot spares as you want. Any
RAID-1 array on an adapter can use the hot-spare disk drives on
that adapter. For full clustering operations to continue, the
hot-spare used by a RAID-1 array must be in an SSA loop between the
same two servers as the array.
If a disk drive within an array fails, after an interval, the
adapter automatically uses a hot spare instead of the failed disk
drive, and rebuilds the data that was on the failed disk on to the hot
spare.
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