Mladen Stefanov F48235 R.A.I.D Data is the most valuable asset of any business today. Lost data, in most cases, means lost business. Even if you backup regularly, you need a fail-safe way to ensure that your data is protected and can be accessed without interruption in the event of an online disk failure. Adding RAID, to your storage configurations is one of the most cost-effective ways to maintain both data protection and access. But what is RAID? RAID is an acronym of “REDUNDANT ARRAY OF INEXPENSIVE DISKS”, defined by D. Patterson, G. Gibson, and R. Katz, a technology that allows, by arranging disks(minimum two) into arrays, users to achieve incredible storage reliability and/or performance, from low-cost and less reliable hard disks drives. There are different schemes/architectures of RAID and they are named by the word RAID followed by a number, as RAID0, RAID1 etc. These various designs, aim several design goals: increase data reliability, increase I/O performance or sometimes both. When several disks are set up to use RAID, they are said to be in RAID array. This array distributes data across multiple disks, but the array is seen by the computer user and operating system as one single disk. Tree key concepts are involved in RAID implementations mirroring, striping and error correction. Mirroring - identical data is written to more than one disk; striping - data is distributed across more than one disk; error correction - redundant ("parity") data is stored to allow problems to be detected and fixed. RAID combines two or more physical hard disks into a single logical unit and can be implemented in hardware, in the form of special disk controllers, or in software, as a kernel module that is layered in between the low-level disk driver, and the file system which sits above it. RAID hardware is always a "disk controller" Hardware-RAID is a device to which one can cable up the disk drives. Usually it comes in the form of an adapter card that will plug into a ISA/EISA/PCI/S- Bus/MicroChannel slot. However, some RAID controllers are in the form of a box
Transcript
Mladen Stefanov F48235
R.A.I.D
Data is the most valuable asset of any business today. Lost data, in most
cases, means lost business. Even if you backup regularly, you need a fail-safe way to
ensure that your data is protected and can be accessed without interruption in the
event of an online disk failure. Adding RAID, to your storage configurations is one of
the most cost-effective ways to maintain both data protection and access. But what
is RAID?
RAID is an acronym of “REDUNDANT ARRAY OF INEXPENSIVE DISKS”, defined
by D. Patterson, G. Gibson, and R. Katz, a technology that allows, by arranging
disks(minimum two) into arrays, users to achieve incredible storage reliability
and/or performance, from low-cost and less reliable hard disks drives. There are
different schemes/architectures of RAID and they are named by the word RAID
followed by a number, as RAID0, RAID1 etc. These various designs, aim several
design goals: increase data reliability, increase I/O performance or sometimes both.
When several disks are set up to use RAID, they are said to be in RAID array. This
array distributes data across multiple disks, but the array is seen by the computer
user and operating system as one single disk. Tree key concepts are involved in RAID
implementations mirroring, striping and error correction. Mirroring - identical data
is written to more than one disk; striping - data is distributed across more than one
disk; error correction - redundant ("parity") data is stored to allow problems to be
detected and fixed.
RAID combines two or more physical hard disks into a single logical unit and
can be implemented in hardware, in the form of special disk controllers, or in
software, as a kernel module that is layered in between the low-level disk driver,
and the file system which sits above it. RAID hardware is always a "disk controller"
Hardware-RAID is a device to which one can cable up the disk drives. Usually
it comes in the form of an adapter card that will plug into a ISA/EISA/PCI/S-
Bus/MicroChannel slot. However, some RAID controllers are in the form of a box
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that connects into the cable in between the usual system disk controller, and the
disk drives. The latest RAID hardware used with the latest & fastest CPU will usually
provide the best overall performance, although at a significant price. This is because
most RAID controllers come with on-board DSP's and memory cache that can off-
load a considerable amount of processing from the main CPU, as well as allow high
transfer rates into the large controller cache. Old RAID hardware can act as a "de-
accelerator" when used with newer CPU's: yesterday's fancy DSP and cache can act
as a bottleneck, and it's performance is often beaten by pure-software RAID and
new but otherwise plain, run-of-the-mill disk controllers. RAID hardware can offer
an advantage over pure-software RAID, if it can makes use of disk-spindle
synchronization and its knowledge of the disk-platter position with regard to the
disk head, and the desired disk-block. However, most modern (low-cost) disk drives
do not offer this information and level of control anyway, and thus, most RAID
hardware does not take advantage of it. RAID hardware is usually not compatible
across different brands, makes and models: if a RAID controller fails, it must be
replaced by another controller of the same type. As of this writing (June 1998), a
broad variety of hardware controllers will operate under Linux; however, none of
them currently come with configuration and management utilities that run under
Linux.
Software-RAID is a set of kernel modules, together with management utilities
that implement RAID purely in software, and require no extraordinary hardware.
The Linux, RAID subsystem is implemented as a layer in the kernel that sits above
the low-level disk drivers (for IDE, SCSI drives and etc.), and the block-device
interface. The file system, ext2fs, DOS-FAT, or whatever we want, sits above the
block-device interface. Software-RAID, by its very software nature, tends to be more
flexible than a hardware solution. The downside is that it of course requires more
CPU cycles and power to run well than a comparable hardware system. Of course,
the cost can't be beat. Software RAID has one further important distinguishing
feature: it operates on a partition-by-partition basis, where a number of individual
disk partitions are ganged together to create a RAID partition. This is in contrast to
most hardware RAID solutions, which gang together entire disk drives into an array.
With hardware, the fact that there is a RAID array is transparent to the operating
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system, which tends to simplify management. With software, there are far more
configuration options and choices, tending to complicate matters.
Stripe width and Stripe size. What are they?
RAID technology use striping to improve performance by splitting up files into
small pieces and distributing them among multiple hard disks. Most striping
implementations allow the creator of the array control over two critical parameters
that define the way that the data is broken into chunks and sent to the various disks.
Each of these factors has an important impact on the performance of a striped
array.
The first parameter is stripe width of the array. Stripe width refers to the
number of parallel stripes that can be written to or red from simultaneously. This is
equal to the number of disks in the array. That means when we adding more drives
to the array, basically we are increasing the parallelism of the array. For example if
we create array of four 160GB drives, we will have greater transfer performance
than two 320GB drives.
The second important parameter is the stripe size. Stripe size, in RAID arrays,
is the smallest allocation unit of logical disk or volume, written to each disk. It is
sometimes called block size or chunk size and values can be from 2kiB to 512kiB. The
impact of stripe size upon performance is more difficult to quantify than the effect
of stripe width but they are two main things we should know.
Decreasing Stripe Size: As stripe size is decreased, files are broken into
smaller and smaller pieces. This increases the number of drives that an average file
will use to hold all the blocks containing the data of that file, theoretically increasing
transfer performance, but decreasing positioning performance.
Increasing Stripe Size: Increasing the stripe size of the array does the opposite
of decreasing it, of course. Fewer drives are required to store files of a given size, so
transfer performance decreases. However, if the controller is optimized to allow it,
the requirement for fewer drives allows the drives not needed for a particular
access to be used for another one, improving positioning performance.
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The improvement in positioning performance that results from increasing
stripe size to allow multiple parallel accesses to different disks in the array depends
entirely on the controller's smarts. For example, some controllers are designed to
not do any writes to a striped array until they have enough data to fill an entire
stripe across all the disks in the array. Clearly, this controller will not improve
positioning performance as much as one that doesn't have this limitation. Also,
striping with parity often requires extra reads and writes to maintain the integrity of
the parity information
It seems like there is no “optimal stripe size”. Everything depends on your
performance needs, what type of data you will store and type of application you run
Operation modes/states
Optimal should be normal operational mode for all RAID arrays. In this state
everything works fine, there is no failed drive, performance is not affected.
Degraded – When malfunction of one or more drives occurs, whole array
enters in degraded state – called by someone “a critical state”. In this mode
performance is reduced and how much it is depends on the type of RAID used, and
also how the RAID controller reacts to the drive failure. One of the reasons for
reduced performance is that one of the drives is no longer available, and the array
must compensate for this loss of hardware. In a two-drive mirrored array, you are
left with an "array of one drive", and therefore, performance becomes the same as
it would be for a single drive. In a striped array with parity, performance is degraded
due to the loss of a drive and the need to regenerate its lost information from the
parity data, on the fly, as data is read back from the array. The other reason is
rebuilding
Rebuilding – this term means restoration process of the array and could be
initiated by two ways, manual or automatically. Automatic rebuilding could be
initiated immediately after hard disk fail occur if there is a dedicated disk, called hot
spare disk and this option in the configuration of array is been enabled. Otherwise
administrator must replace failed disk and start rebuilding process manually. A
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mirrored array must copy the contents of the good drive over to the replacement
drive. Rebuilding process are going to be time-consuming and also relatively slow - it
can take several hours. During this time, the array will function properly, but its
performance will be greatly diminished. The impact on performance of rebuilding
depends entirely on the RAID level and the nature of the controller, but it usually
affects it significantly. Hardware RAID will generally do a faster job of rebuilding
than software RAID. Fortunately, rebuilding doesn't happen often.
Degraded and Rebuilding state, both are critical states because there is no
data protection and array has no fault tolerance
STANDART RAID LEVELS
RAID 0 (Striping)
Offers low cost and maximum performance, but offers no fault tolerance; a
single disk failure results in TOTAL data loss. Businesses use RAID 0 mainly for tasks
requiring fast access to a large capacity of temporary disk storage (such as
video/audio post-production, multimedia imaging, CAD, data logging, etc.) where in
case of a disk failure, the data can be easily reloaded without impacting the
business. There are also no cost disadvantages as all storage is usable. RAID 0 usable
capacity is 100% as all available drives are used.
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RAID 1 (Mirroring)
Provides cost-effective, high fault tolerance for configurations with two disk
drives. RAID 1 refers to maintaining duplicate sets of all data on separate disk drives.
It also provides the highest data availability since two complete copies of all
information are maintained. There must be two disks in the configuration and there
is a cost disadvantage as the usable capacity is half the number of available disks.
RAID 1 offers data protection insurance for any environments where absolute data
redundancy, availability and performance are key, and cost per usable gigabyte of
capacity is a secondary consideration.
RAID 1 usable capacity is 50% of the available drives in the RAID set.
RAID 2 (Hamming code parity)
Seldom used anymore, and to some degree are have been made obsolete by
modern disk technology. RAID-2 is similar to RAID-4, but stores ECC information
instead of parity. Since all modern disk drives incorporate ECC under the covers, this
offers little additional protection. RAID-2 can offer greater data consistency if power
is lost during a write; however, battery backup and a clean shutdown can offer the
same benefits. RAID-2 is not supported by the Linux Software-RAID drivers.
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RAID 3 (Striped set with dedicated parity)
This mechanism provides fault tolerance similar to RAID 5. However, because
the stripe across the disks is much smaller than a file system block, reads and writes
to the array perform like a single drive with a high linear write performance. For this
to work properly, the drives must have synchronized rotation. If one drive fails,
performance is not affected.
RAID 4 (Block level parity)
Interleaves stripes like RAID-0, but it requires an additional partition to store
parity information. The parity is used to offer redundancy: if any one of the disks fail
the data on the remaining disks can be used to reconstruct the data that was on the
failed disk. Given N data disks, and one parity disk, the parity stripe is computed by
taking one stripe from each of the data disks, and XOR'ing them together. Thus, the
storage capacity of a an (N+1)-disk RAID-4 array is N, which is a lot better than
mirroring (N+1) drives, and is almost as good as a RAID-0 setup for large N. Note
that for N=1, where there is one data drive, and one parity drive, RAID-4 is a lot like
mirroring, in that each of the two disks is a copy of each other. However, RAID-4
does NOT offer the read-performance of mirroring, and offers considerably
degraded write performance. In brief, this is because updating the parity requires a
read of the old parity, before the new parity can be calculated and written out. In an
environment with lots of writes, the parity disk can become a bottleneck, as each
write must access the parity disk.
RAID 5 (Striping with parity)
Uses data striping in a technique designed to provide fault-tolerant data
storage, but doesn't require duplication of data like RAID 1 and RAID 1E. Data is
striped across all of the drives in the array, but for each stripe through the array
(one stripe unit from each disk) one stripe unit is reserved to hold parity data
calculated from the other stripe units in the same stripe. Read performance is
therefore very good, but there is a penalty for writes, since the parity data has to be
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recalculated and written along with the new data. To avoid a bottleneck, the parity
data for consecutive stripes is interleaved with the data across all disks in the array.
RAID 5 has been the standard in server environments requiring fault
tolerance. The RAID parity requires one disk drive per RAID set, so usable capacity
will always be one disk drive less than the number of available disks in the
configuration of available capacity - still better than RAID 1 which as only a 50%
usable capacity. RAID 5 requires a minimum of three disks and a maximum of 16
disks to be implemented. RAID 5 usable capacity is between 67% - 94%, depending
on the number of data drives in the RAID set.
RAID 6 (Striping with dual parity)
Data is striped across several physical drives and dual parity is used to store
and recover data. It tolerates the failure of two drives in an array, providing better
fault tolerance than RAID 5. It also enables the use of more cost-effective ATA and
SATA disks to storage business critical data. This RAID level is similar to RAID 5, but
includes a second parity scheme that is distributed across different drives and
therefore offers extremely high fault tolerance and drive failure tolerance. RAID 6
can withstand a double disk failure. RAID 6 requires a minimum of four disks and a
maximum of 16 disks to be implemented. Usable capacity is always 2 less than the
number of available disk drives in the RAID set. With less expensive, but less reliable
SATA disk drives in a configuration that employs RAID 6, it is possible to achieve a
higher level of availability. This is because the second parity drive in the RAID 6 RAID
set can withstand a second failure during a rebuild. In a RAID 5 set, the degraded
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state and/or the rebuilding time onto a hot spare is considered the window at which
the RAID array is most vulnerable to data loss. During this time, if a second disk
failure occurs, data is unrecoverable. With RAID 6 there are no windows of
vulnerability as the second parity drive protects against this.
NON-STANDART RAID LEVELS
RAID 1E (Striped Mirroring)
Combines data striping from RAID 0 with data mirroring from RAID 1. Data
written in a stripe on one disk is mirrored to a stripe on the next drive in the array.
The main advantage over RAID 1 is that RAID 1E arrays can be implemented using an
odd number of disks. When using even numbers of disks it is always preferable to
use RAID 10, which will allow multiple drive failures. With odd numbers of disks,
however, RAID 1E supports only one drive failure. RAID 1E usable capacity is 50% of
the total available capacity of all disk drives in the RAID set.
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RAID 5EE (Hot Space)
Provides the protection of RAID 5 with higher I/Os per second by utilizing one
more drive, with data efficiently distributed across the spare drive for improved I/O
access. RAID 5EE distributes the hot-spare drive space over the N+1 drives
comprising the RAID-5 array plus standard hot-spare drive. This means that in
normal operating mode the hot spare is an active participant in the array rather
than spinning unused. In a normal RAID 5 array adding a hot-spare drive to RAID 5
array protects data by reducing the time spent in the critical rebuild state. This
technique does not make maximum use of the hot-spare drive because it sits idle
until a failure occurs. Often many years can elapse before the hot-spare drive is ever
used. For small RAID 5 arrays in particular, having an extra disk to read from (four
disks instead of three, as an example) can provide significantly better read
performance. For example, going from a 4-drive RAID 5 array with a hot spare to a
5-drive RAID 5EE array will increase performance by roughly 25%. One downside of
RAID 5EE is that the hot-spare drive cannot be shared across multiple physical arrays
as with standard RAID 5 plus hot-spare. RAID 5 technique is more cost efficient for
multiple arrays because it allows a single hot-spare drive to provide coverage for
multiple physical arrays. This configuration reduces the cost of using a hot-spare
drive, but the downside is the inability to handle separate drive failures within
different arrays. This RAID level can sustain a single drive failure. RAID 5EE useable
capacity is between 50% - 88%, depending on the number of data drives in the RAID
set. RAID 5EE requires a minimum of four disks and a maximum of 16 disks to be
implemented.
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NESTED(hybrid) RAIDs
RAID 10 (Striping and mirroring)
Combines RAID 0 striping and RAID 1 mirroring. This level provides the
improved performance of striping while still providing the redundancy of mirroring.
RAID 10 is the result of forming a RAID 0 array from two or more RAID 1 arrays. This
RAID level provides fault tolerance - up to one disk of each sub-array may fail
without causing loss of data. Usable capacity of RAID 10 is 50% of available disk
drives.
RAID 50 (Striping)
Combines multiple RAID 5 sets with RAID 0 (striping). Striping helps to
increase capacity and performance without adding disks to each RAID 5 array (which
will decrease data availability and could impact performance when running in a
