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Oracle Tips by Burleson |
Disk Shadowing or Mirroring
If you will have mission-critical
applications that you absolutely cannot allow to go down, consider
disk shadowing or mirroring. As the term implies, disk shadowing or
mirroring is the process whereby each disk has a shadow or mirror
disk to which data is written simultaneously. This redundant storage
allows the shadow disk or set of disks to pick up the load in case
of a disk crash on the primary disk or disks; thus the users never
see a crashed disk. Once the disk is brought back online, the shadow
or mirror process brings it back in sync by a process appropriately
called resilvering. This also allows for backup since the shadow or
mirror set can be broken (e.g., the shadow separated from the
primary), a backup taken, and then the set resynchronized. I have
heard of two, three, and even higher-number mirror sets. Generally,
I see no reason for more than a three-way mirror as this allows for
the set of three to be broken into a single and a double set for
backup purposes. Shadowing or Mirroring is RAID1.
The main disadvantage to disk shadowing is
the cost: For a 200-GB disk “farm,” you need to purchase 400 or more
gigabytes of disk storage.
Redundant Arrays of Inexpensive Disks (RAID)
The main strength of RAID technology is its
dependability. In a RAID5 array, the data is stored, as is parity
data and other information about the contents of each disk in the
array. If one disk is lost, the others can use this stored
information to re-create the lost data. This makes RAID 5 very
attractive. RAID 5 has the same advantages as shadowing and striping
but at a lower cost. It has been suggested that if the manufacturers
would use slightly more expensive disks (RASMED—redundant array of
slightly more expensive disks) performance gains could be realized.
A RAID 5 system appears as one very large, reliable disk to the CPU.
There are several levels of RAID to date:
RAID0. Known as disk striping.
RAID1. Known as disk shadowing.
RAID0/1. Combination of RAID0 and RAID1.
May also be called RAID10 depending on whether they are striped and
mirrored or mirrored then striped. It is generally felt that RAID10
performs better than RAID01.
RAID2. Data is distributed in extremely
small increments across all disks and adds one or more disks that
contain a Hamming code for redundancy. RAID2 is not considered
commercially viable due to the added disk requirements (10 to 20
percent must be added to allow for the Hamming disks).
RAID3. This also distributes data in
small increments but adds only one parity disk. This results in good
performance for large transfers; however, small transfers show poor
performance.
RAID4. In order to overcome the small
transfer performance penalties in RAID3, RAID4 uses large data
chunks distributed over several disks and a single parity disk. This
results in a bottleneck at the parity disk. Due to this performance
problem, RAID4 is not considered commercially viable. RAID3 and -4
are usually are used for video streaming technology or large LOB
storage.
RAID5. This solves the bottleneck by
distributing the parity data across the disk array. The major
problem is that it requires several write operations to update
parity data. That said, the performance hit is only moderate, and
the other benefits may outweigh this minor problem. However, the
penalty for writes can be over 20 percent and must be weighed
against the benefits.
RAID6. This adds a second redundancy disk
that contains error-correction codes. Read performance is good due
to load balancing, but write performance suffers because RAID6
requires more writes than RAID5 for data update.
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