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| Oracle Storage Subsystems |
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| RAID Level |
Description |
| 0 |
Striping: The easy way to remember
that is that RAID 0 adds no redundancy. In fact, if you stripe disks to
improve performance, you will have worse availability than if you just use
disks without RAID. That's because if you stripe, the loss of any of the
disks in the stripe will cause the entire stripe to fail. |
| 1 |
Mirroring.
All of the data on one disk is copied exactly onto a second disk. Neither
disk is the master or primary; the disks are peers. For writes to be
deemed complete, they must make it to both disks. If one disk fails, its
partner keeps right on running, without interruption. The good news with
RAID-1 is that it's very easy to manage and it does not require
significant levels of CPU for normal operations or for recovery. The
downside to RAID-1 is the expense: For every gigabyte of disk you wish to
protect, you need a second, matching gigabyte. In other words, RAID-1
requires twice as much disk space as unprotected disks. |
| 2 |
Hamming code error correction.
RAID-2 uses the same hamming encoding method for checking the correctness
of disk data that error correcting code memory (ECC) uses. I have never
seen or heard of a single commercial implementation of RAID-2. I include
it here only for completeness and because if I left it out, someone would
write to ask me about it. |
| 3 |
Virtual disk blocks.
In RAID-3, every write is split (striped) across all of the disks (usually
four or more) in the RAID array. Since every write touches every disk, the
array can only be writing one block of data at a time that can cause poor
performance from the RAID. RAID-3 performance varies based on the nature
of the writes: Small writes scattered all over the disks will show very
poor performance. Larger sequential writes will result in better
performance. |
| 4 |
Dedicated parity disk.
In a RAID-4 array, there is a set of data disks, usually 4 or 5 (although
there could be more, at a significant performance penalty), plus one extra
disk that is dedicated to managing the parity for the data on the other
disks. Since all writes must go through the parity disk, that disk becomes
a performance bottleneck slowing down all write activity to the entire
array. |
| 5 |
Striped parity.
RAID-5 is virtually identical to RAID-4 except that instead of all of the
parity being concentrated on a single disk, the parity is divided up, with
a share being given to each disk in the array. This sharing will balance
and reduce the performance impact that is evident in RAID-4
implementations. In software implementations of RAID-5, which are fairly
common, performance will often become unacceptably slow if writes make up
any more than about 15% of disk activity. |
| 6 |
Dual parity.
RAID-6 takes the striped parity region from RAID-5 and duplicates it. Each
disk in a RAID-6 stripe has two parity regions, each calculated
separately. The advantage of RAID-6 is that even if two disks fail, the
RAID stripe would still have a complete set of data and be able to recover
the failed disks. The biggest disadvantage is in performance. The
performance impact of RAID-5 is roughly doubled as each set of parities is
calculated separately. Where RAID-5 requires one extra disk for parity
RAID-6 requires two, increasing the cost of the implementation. |
| 7 |
RAID-7 is a trademarked term, owned by
Storage Computer Corporation. They appear to have the only commercial
implementation that involves proprietary disk array hardware with an
internal CPU and a combination of striping and a RAID-5-like model. |
| S |
EMC Symmetrix disk arrays offer an
alternate, proprietary method for parity RAID that they call RAID-S. While
detailed documentation on RAID-S is proprietary, it appears to be similar
to RAID-5 with some performance enhancements as well as the enhancements
that come from having a high-speed disk cache on the disk array. |
| 5+0 |
By building a series of RAID-5 groups and
then striping them in RAID-0 fashion, you can improve RAID-5 performance
without reducing data protection. |
| 5+3 |
It would be more accurate to call this RAID
0+3 but it has come to be called RAID 5+3 and it's defined by striping (in
RAID-0 style) RAID-3's virtual disk blocks. |
| 10 |
Combining RAID-0 and RAID-1 is often
referred to as RAID-10, and it comes in two different forms. In RAID-0+1,
the data is organized as stripes across multiple disks, and then the
striped disk sets are mirrored. In RAID-1+0, the data is mirrored and the
mirrors are striped. |
|
| |
| Drive # |
RAID Configuration |
| 1 |
2 |
3 |
4 |
JBOD |
RAID 0 |
RAID 1 |
RAID 0+1 |
RAID 1+0 |
RAID 5 |
| O |
O |
O |
O |
online |
online |
online |
online |
online |
online |
|
Single Drive Failure |
| O |
O |
O |
X |
fail |
fail |
online |
online |
online |
online |
| O |
O |
X |
O |
fail |
fail |
online |
online |
online |
online |
| O |
X |
O |
O |
fail |
fail |
online |
online |
online |
online |
| X |
O |
O |
O |
fail |
fail |
online |
online |
online |
online |
|
Two Drive Failures |
| X |
X |
O |
O |
fail |
fail |
online |
online |
fail |
fail |
| X |
O |
X |
O |
fail |
fail |
online |
fail |
online |
fail |
| X |
O |
O |
X |
fail |
fail |
online |
fail |
online |
fail |
| O |
X |
X |
O |
fail |
fail |
online |
fail |
online |
fail |
| O |
X |
O |
X |
fail |
fail |
online |
fail |
online |
fail |
| O |
O |
X |
X |
fail |
fail |
online |
online |
fail |
fail |
|
Three Drive Failures |
| X |
X |
X |
O |
fail |
fail |
online |
fail |
fail |
fail |
| X |
X |
O |
X |
fail |
fail |
online |
fail |
fail |
fail |
| X |
O |
X |
X |
fail |
fail |
online |
fail |
fail |
fail |
| O |
X |
X |
X |
fail |
fail |
online |
fail |
fail |
fail |
|
Four Drive Failures |
| X |
X |
X |
X |
fail |
fail |
fail |
fail |
fail |
fail |
|
| |
X' means the drive failed, while 'O' means the drive is online.
In RAID 0+1, drives in pairs (1,2) and (3,4) are striped, and then the pairs are mirrored.
In RAID 1+0 or 10, drives in pairs (1,2) and (3,4) are mirrored, and then the pairs are striped. |
|
| |
JBOD |
The simplest way to configure a disk drive array is to combine multiple disks together so
that they look like a single logical drive to the system. This technique can be used where disk space is at a premium
and there are many drives of varying capacities to be used. The total capacity is the sum of all the drives, and the
performance is equal to the performance of the drive that the controller is accessing. If a single disk fails the entire
array goes offline. The data becomes unrecoverable even though data files may be intact on the remaining drives. This
most basic RAID configuration is called Just a Bunch of Disks (JBOD), and is supported by virtually every RAID controller
including the FastTrak SX4.
A way to increase performance is to configure the controller so that it stores data in stripes across two or more drives.
With data split up across the drives, reads and writes can occur faster because the controller can read from and write to
multiple drives at once. Under certain ideal conditions, the reading and writing can occur at a speed proportional to the
number of drives in the array. For example, with two drives, one can achieve twice the performance of a single drive under
certain conditions. With a striped array, the total capacity is the capacity of the smallest drive times the number of
drives. For example, two striped 80GB drives form a single, faster 160GB drive. A major drawback of striping is that a single
drive failure causes total data loss. The remaining drives only contain pieces of files, and there is no software with the
controller to recover even the pieces. The striped drive configuration is called RAID-0 (pronounced raid zero).
One way to increase reliability is to configure the controller so that it mirrors data to two or more drives to make a single
logical drive with data redundancy. If one physical drive fails, a perfect mirror of the data will be on the surviving physical
drives, and you can continue using the data without interruption. After a replacement is drive is installed, the array can be
rebuilt by the controller, which means copying the data from the surviving drives onto the replacement drive. Because data is
written to and read from synchronously, the performance is no faster than the slowest drive in the array. The total amount of
storage available in a mirrored array will be limited to the capacity of the smallest drive, regardless of how many drives are
mirrored. Mirrored drives are said to be in a RAID-1 (pronounced raid one) configuration. |
|
|
Hard drives are mechanical devices and susceptible to many kinds of failure.
- manufacturing defects
- mishandling during shipping or installation
- EMI, magnetic fields, static electricity
- operating environment - power fluctuations, temperature, vibration, shocks, bad software
- wear and tear - seeking, spinning, on/off cycling
Striping drives significantly decreases reliability, more so than commonly expected. Striped arrays should not be used to
store important information because there is a very high probability of failure. On the other hand, mirroring drives
significantly increases reliability, and a two drive mirrored array will tend to survive a long time. Precise calculations
of reliability in multi-drive arrays is complex enough that teams at IBM research it, and an alternative would be a policy
of preparing for the worst and then deriving reliability information from experience in the particular environment at hand.
For more information, you can read the Hard Disk Reliability Article and try their online Calculator. In the example at the
end of the article, the three disk RAID-0 array is 10,000 times more likely to fail in a given month than the three disk
RAID-1 array. In fact, the odds of the three disk RAID-0 array failing in a given month are an incredible one in twenty.
Remember that other parts of the system can fail besides just the drives, including the controllers and the cables themselves.
|
|
Both RAID-0 and RAID-1 require the synchronization of disk drive heads. Seek time
of a disk drive is the time it takes for the read/write head to position itself over the right track on the disk, and for the
disk to spin to the position of the data. If disks are of different speeds, then seek time in an array will be lowered to that
of the slowest drive. Seek time is important for applications that require reading and writing to many random locations, such
as file servers. Neither RAID-0 nor RAID-1 have any effect on seek time. Seek time can be improved by faster spinning disks or
disks with greater data density.
Another aspect of drive performance is sustained transfer rate. This is the amount of contiguous data that can be read or
written over a certain period of time. Since it is usually determined for large blocks of data, it indicates performance for
applications that manipulate large data files, such as image and video files. Striping can multiply sustained transfer rate
by the number of drives in the striped array, whereas mirroring has no effect on transfer rate. Windows XP has a feature
called disk optimization and application/boot prefetching, which also benefits significantly from striping. This feature
basically reorganizes the files into larger, contiguous blocks, speeding up bootup and program startup. Of course, there are
more aspects to performance than just these basic ones.
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