5.What is Mirroring?
Mirroring involves having two copies of the
same data on separate hard drives or drive arrays. So basically
the data is effectively mirrored on another drive. The system
basically writes data simultaneously to both hard drives.
This is one of the two data redundancy techniques used in
RAID to protect from data loss. The benefit is that when one
hard drive or array fails, the system can still continue to
operate since there are two copies of data. Downtime is minimal
and data recovery is relatively simple. All you need to do
is rebuild the data from the good copy.
The RAID controller writes the same data blocks
to each mirrored drive. Each drive or array has the same information
in it. You can even add another level of complexity by introducing
striping. If you have one striped
array, you can mirror the array at the same time on a second
striped array. It can get very complicated. To set up mirroring
the number of drives will have to be doubled for obvious reasons.
The drawback here is that both drives are
tied up during the writing process which limits parallelism
and can hurt performance. In contrast, mirroring has a performance
increase when dealing with reads. A good RAID controller will
only read from one of the drives since the data on both are
the same. While the other is used to read, the free drive
can be used for other requests. This increases parallelism,
which is pretty much the concept behind the performance increase
Mirroring may seem like a good solution to ensure the safety
of data. However, the trade off here is the cost and wasted
space involved with having two copies of the same data. You
will need twice as much storage space to mirror the data.