RAID
1 Data Recovery
RAID 1 creates
an exact copy
(or mirror) of
a set of data
on two or more
disks. This is
useful when write
performance is
more important
than minimizing
the storage capacity
used for redundancy.
This is thought
to be a foolproof
method of data
protection, but
we commonly receive
RAID 1 arrays
that have failed
due to:
- corrupted
mirrors
- bad data
from one drive
moves to the
other drive
- mirror breaks,
and does not
allow system
to boot
- improper
rebuild
The array can
only be as big
as the smallest
member disk,
however. A classic
RAID 1 mirrored
pair contains
two disks, which
increases reliability
by a factor
of two over
a single disk,
but it is possible
to have many
more than two
copies. Since
each member
can be addressed
independently
if the other
fails, reliability
is a linear
multiple of
the number of
members. To
truly get the
full redundancy
benefits of
RAID 1, independent
disk controllers
are recommended,
one for each
disk. Some refer
to this practice
as splitting
or duplexing.
When reading,
both disks can
be accessed
independently.
Like RAID 0
the average
seek time is
reduced by half
when randomly
reading but
because each
disk has the
exact same data
the requested
sectors can
always be split
evenly between
the disks and
the seek time
remains low.
The transfer
rate would also
be doubled.
For three disks
the seek time
would be a third
and the transfer
rate would be
tripled. The
only limit is
how many disks
can be connected
to the controller
and its maximum
transfer speed.
Many older IDE
RAID 1 cards
read from one
disk in the
pair, so their
read performance
is that of a
single disk.
Some older RAID
1 implementations
would also read
both disks simultaneously
and compare
the data to
catch errors.
The error detection
and correction
on modern disks
makes this less
useful in environments
requiring normal
commercial availability.
When writing,
the array performs
like a single
disk as all
mirrors must
be written with
the data.
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