With the recent discussion of servers, I thought I would pass on the most recent Epicor/Progress statement on the discussion of "which RAID array is best for Vantage". The clear answer is RAID 1. The reasons are discussed below in an Epicor document on the subject.
Troy Funte
Liberty Electronics
Summary: XA RAID 0, RAID 1, and RAID 5 Explained
Book: Support Solutions
Page: 130.1730MPS
PROBLEM DESCRIPTION:
Which is better, RAID 0, RAID 1, or RAID 5 for the dedicated server?
Why should or shouldn't I use mirroring or RAID drives?
PROBLEM RESOLUTION:
RAID 1 is the RAID level of choice, as it provides the best performance and
data
reliability for use with a Progress database.
RAID support usually comes in three forms: operating system software,
controller
level support and dedicated RAID devices. One advantage to using software
RAID
is that you can break the mirror and backup a static copy of your database,
re-mirroring after backup is complete. The disadvantage is that software is
subject to competition with the rest of the processes running on the
computer
and can adversely impact performance.
RAID 0 (striping) works well with a Progress database and offers improved
performance by reducing head seek time.
RAID 1 (mirroring) works exceptionally well with a Progress database. Since
it
provides complete data redundancy, it is a very reliable solution that also
offers good read/write I/O performance.
**RAID 5 is not recommended by Epicor Software Corporation or by Progress.**
RAID 5 is combination of striping and parity. It offers the data
reliability
approaching that of mirroring, with the read I/O performance of striping.
However, there is a substantial performance penalty when write I/O
operations
are done. The Vantage application is extremely write I/O intensive. For
that
reason, RAID 5 IS NOT RECOMMENDED for use with a Progress database. As your
database grows in size, the performance penalty will become more apparent
over
time.
Additionally, a RAID 5 box with its own controller often has on-board write
caching that can subvert Progress write integrity algorithms, and can cause
database corruption. Make sure that any write caching is backed up by some
UPS mechanism.
As a final comment, RAID that is implemented within the NT operating system,
with the server's CPU handling write caching, can be dangerous to data
integrity; unlike dedicated RAID boxes, there isn't a reliable mechanism for
recovering write-cached data that wasn't yet written if the server goes
down.
====================================================
RAID OPTIONS - Benefits and Drawbacks.
Hard Disk Fault Tolerance
When more than one physical disk is used by the server, it is possible to
configure them in a 'stripe set' or a 'mirrored set' that is intended to
improve the speed of disk operations and to provide for tolerance in the
case of drive failure. This is a complex area, because the striping or
mirroring can be controlled by dedicated hardware, the operating system, or
even disk controller drivers. Each has merits under different circumstances.
Also, it is possible to include redundant drives that offer a certain level
of fault tolerance (useful for a mission critical system). RAID 1, RAID 5
and RAID 10 all provide for fault tolerance.
RAID Levels
==========
The RAID concept (Redundant Arrays of Inexpensive Disks) was first proposed
in 1987 by researchers Patterson, Gibson, and Katz at the University of
California at Berkeley. They proposed that the cost, performance, size
limitations, and reliability of storage subsystems could be improved by
combining several small disks into a disk "array" that is perceived as a
single disk by applications (and the operating system).
They described five ways of combining disks, numbered 1 through 5. Later,
RAID 0 was added to the list. Combination RAID levels are also possible to
an extent, RAID10 is
a combination on Raid 1 (Mirrors) and RAID 0 (Striping), in this case the
benefits of each RAID type are exploited. Each configuration, however, has
different costs, benefits, and drawbacks.
RAID levels 2, 3 and 4 are not commercially viable, and are excluded from
this document.
RAID 0
======
RAID level 0 is also called "striping". In this configuration, a group of
disks act as a single disk (a "stripe set") with data striped across all the
disks. The striping strategy is that the total storage space divided into
equal-size sections or "stripe blocks" that are allocated round-robin among
all the disks. The size of the striper block is configurable is normally a
megabyte long or longer. Technically RAID0 is not RAID at all, as there is
no redundancy.
** BENEFITS **
· Read and write performance for large sequential files is greatly
enhanced because all the disks in the stripe set can be accessed in
parallel. Read and write performance for small random access transfers is
about the same as for separate disks, but under special circumstances can
actually be worse.
· Disk activity will, on the average, be balanced across all the disks
in
the stripe set so no one disk will become a "hot spot" with much more
activity than the others. This improves performance on the average for all
users.
· Several small disks can be combined into a single large logical disk
to
provide increased storage capacity.
** DRAWBACKS**
· Reliability is extremely low. Failure of a single disk causes loss
of the entire stripe set. The mean time to failure (mttf) of the entire
stripe set is inversely proportional to the number of disks.
· Small file handling can be cumbersome if the file sizes are slightly
larger than the stripes.
RAID 1
RAID level 1 is also called "mirroring". In this configuration, a duplicate
copy of each disk is stored on a second "mirror" drive. All data are written
to both sides of the mirror. This provides redundancy and fault tolerance
because each disk is duplicated. If one fails, its mirrors can still be
accessed and no data are lost because of the failure. One can configure RAID
1 to have more than one duplicate (e.g. "triple mirroring") copy of each
disk; this is generally regarded as overkill however.
** BENEFITS **
· Reliability is very high.
· A disk failure has no impact on WRITE performance.
· If any disk fails, its mirror can continue to function and the
failed disk
can be replaced when it is convenient. Many modern storage subsystems are
designed so that failed components can be replaced while the storage
subsystem is fully operational.
· Read performance is better than single drives because data can be
read from either side of the mirror.
** DRAWBACKS**
· Cost is highest. The cost per megabyte is slightly more than doubled
(or tripled if two redundant copies are made) compared to single drives. On
the other hand, disks are increasingly inexpensive thereby offsetting this
issue to a large extent.
· There can be a very slight degradation in write performance compared
to single drives.
RAID 5
======
In RAID level 5 configurations, data are striped across several disks along
with "parity" data. The striping strategy is the same as for RAID 0
(relatively large stripe blocks). The parity data is distributed across the
drives in such a way that a data group and its parity information
are always written to different devices. This technique allows
reconstruction of all data that was present on any single drive that has
failed. Some implementations allow the parity drive to be a single drive,
others allow this parity information to be stored on the other drives in the
array.
** BENEFITS **
· READ performance can be quite good, especially where multiple disks
are RAID'ed into an array.
· Reliability will be high. If one disk fails, the remaining disks
continue to function without loss of data or availability. The failed disk
can be replaced when it is convenient.
· Cost is lower when compared with mirroring (RAID 1), a typical RAID5
array consists of three disks, giving 2/3 availability of the storage
purchased (3 x 18Gb drives would give around 36GB). ( Note: You can setup
RAID-5 arrays of up to seven drives with hotspares included. )
** DRAWBACKS**
· WRITE performance has the potential to be terrible for database
based
applications. This is because the parity data must be updated whenever a
block is written. In the worst case, writing a single database block
requires four or more (in the case of a block being split over stripes) i/o
operations. The following operations are required:
1. Read the old stripe block
2. Read the old parity data
3. Exclusive or the old data group with the parity data
4. Merge the new database block into the old data group
5. Exclusive or the new data with the parity data
6. Write the new stripe block
7. Write the new parity data
"But since the data and parity are on separate drives, they can be read in
parallel" you say. Whilst this is true, reading two disks at the same time
uses up half your disk bandwidth. As you
can see, writing can use even more than half, giving the potential for
serious WRITE performance concerns.
· Although a failed disk can be replaced, when they are,
reconstructing its
contents requires reading the entire contents of every other disk, writing
the entire contents of the failed disk, and rewriting the parity information
on all other disks. This can adversely affect overall
system performance whilst this operation takes place.
· In the case of a single Parity disk on low-cost RAID5
implementations,
this disk is often considered to be a point failure problem.
RAID 10
=======
RAID level 10 is a combination of Mirroring and Striping. This takes a
minimum of four identical drives to implement, and gives all of the
advantages of both the striping and mirroring concepts, and reduces the
disadvantages. In this configuration, a duplicate copy of each pair of
striped disks is stored on a second pair of mirror drives. All data are
written to both sides of the mirror. This provides redundancy and fault
tolerance because each disk is duplicated. If one fails, its mirrors can
still be accessed and no data are lost because of the failure. This provides
the high performance aspects of Mirrored READ's, and striped WRITE's, In a
multiple disk environment this costs no more that RAID1, and only 1/3 more
than RAID5..
** BENEFITS **
· Reliability is very high. A disk failure can have almost no impact
on
performance.
· If any disk fails, its mirror can continue to function and the
failed disk
can be replaced when it is convenient. Many modern storage subsystems are
designed so that failed components can be replaced while the storage
subsystem is fully operational (Hot Swap).
· Read and write performance for large sequential files (such as those
that databases use) is greatly enhanced because all the disks in the stripe
set can be accessed in parallel.
· Read and write performance for small random access transfers is
about twice as good as for separate disks, whilst over-all read performance
can be three or more times that of a single drive.
· Disk activity will, on the average, be balanced across all the disks
in
the stripe set so no one disk will become a "hot spot" with much more
activity than the others. This improves
performance on the average for all users.
** DRAWBACKS**
· Cost is high due to the mirroring, 50% of the hardware is redundant.
However disks are no longer expensive items.
· You cannot add "Another 18GB" without buying two 18GB disks, seems a
little odd to many people.
Note: If you are contemplating RAID 1, and wish to have multiple disks, RAID
10 will give you a performance boost over two "straight" RAID1 mirrored
implementations.
Implementation types
There are four common RAID implementations: in the operating system, in the
disk controller, via a dedicated RAID controller hardware or in a separate
subsystem. Separate disk subsystems are discussed later in this document,
but summarized here for the sake of clarity.
A RAID implementation that is part of the operating system is cheap - you
don't have to pay many dollars for it (or not much). It isn't worth much
either and should be avoided wherever possible, the additional load put on
the server, and the lack of hardware level control makes this very
undesirable.
RAID implementation in a disk controller uses less processor power and
system resources than an operating system based solution. Usually, special
device drivers are required for these disk controllers. These types of
systems are inexpensive but provide very limited fault-tolerance. Failed
disks cannot be replaced while the system is operational. This is however;
better than no RAID at all.
RAID implementation via a dedicated RAID controller card, combined with
hot-swappable disks. These are finding their way onto even entry-level
servers, and give the advantage of not having to take the machine off-line
to replace a disk; they are a very cost effective method of adding RAID to a
server.
A RAID implementation that is a separate subsystem from the computer is
best. It can have its own power supplies, battery backup, spare controllers,
hot-standby disks, and other capabilities all of which are independent of
the computer. Failure of a component in the computer should
not affect the disk subsystem and vice versa. An excellent example of this
type of system is Data General's Clariion disk array. Many other vendors
also provide these types of systems. This is discussed in more detail later
in this document.
RAID Recap
===========
RAID 0, or disk striping, is a simple disk striping strategy. Data is
written evenly across all disks in the array providing optimal read and
write performance as operations are performed across multiple disks. A major
disadvantage of RAID 0 is that it does not provide any fault tolerance and
all data is lost if one disk in the array fails. RAID 0 offers high
performance READ and WRITES, but has no fault tolerance. This type of array
is suitable for often accessed temporary files. We do not advise that RAID 0
is used to store important data. This is not technically RAID, as there is
no redundancy.
RAID 1, or mirroring, provides excellent fault tolerance as data is written
to both disks on a mirrored pair and each is a working copy of the data.
Writing to a mirrored pair takes no longer than writing to a single disk and
good disk controllers can read data almost twice as fast by reading from
both disks in parallel. However, RAID 1 implementations result in a loss of
50% of disk space that you have purchased, as data is written twice.
RAID 5, or disk stripping with parity, extends the concept of disk stripping
and uses parity to provide fault tolerance at the cost of additional reads
and writes when data blocks are updated. Data redundancy is provided by the
parity information. Although RAID 5 write performance is
substantially slower than disk striping without parity (RAID 0), striping
with parity generally offers similar READ performance to disk mirroring
(RAID 1). Disk space is needed for the parity information, rendering some
portion of disk space unusable for data. However, this loss is less than
that of RAID 1 implementations. This is a RAID level that is promoted by
many hardware venders as it is less costly that RAID1/10 whilst still
offering high levels of data security.
RAID 10 (RAID 1 + RAID 0, or RAID0+1/RAID1+0)) is also known as mirroring
with striping. This level uses a striped array of disks, which are then
mirrored to another identical set of striped disks. For example, a striped
array can be created using three disks. The striped array of disks
is then mirrored using another set of three striped disks. RAID 10 provides
the performance benefits of disk striping with the disk redundancy of
mirroring, without the performance issues associated with parity
maintenance. RAID 10 provides the highest read/write performance of any of
the RAID levels at the expense of using twice as many disks. This is advised
for high security and high performance situations.
External Disk Systems. This includes such esoteric systems as the Clariion
disk array. This is a separate subsystem that is not a physical part of the
your server. They are very fast, very secure, and often highly expensive.
They are currently the ultimate in speed and safety, and
highly recommended for enterprise level servers
With redundancy, RAID 1, 5 and 10 implementations allow for the server to
remain operational even if a drive fails. Some drive arrays also allow for
repair and rebuild of the failed drive while the server continues
uninterrupted operations, often automatically, other systems (usually less
expensive ones) demand that the server is taken off line whilst this rebuild
is accomplished. Again, these issues are complex, and cannot be thoroughly
covered by this document; they should be discussed with your hardware vendor
and implementation team.
Hardware-Based Disk Cache
=======================
Systems that contain a large READ disk cache within the controller can
dramatically improve the performance of the application server. This option
is relatively inexpensive when purchased with the overall system and
therefore is recommended, particularly for larger implementations. WRITE
caches can cause integrity issues under extremely rare, and complex
circumstances. Most caches can be configured easily enough to avoid this
rare issue by turning off the write cache.
[Non-text portions of this message have been removed]
Troy Funte
Liberty Electronics
Summary: XA RAID 0, RAID 1, and RAID 5 Explained
Book: Support Solutions
Page: 130.1730MPS
PROBLEM DESCRIPTION:
Which is better, RAID 0, RAID 1, or RAID 5 for the dedicated server?
Why should or shouldn't I use mirroring or RAID drives?
PROBLEM RESOLUTION:
RAID 1 is the RAID level of choice, as it provides the best performance and
data
reliability for use with a Progress database.
RAID support usually comes in three forms: operating system software,
controller
level support and dedicated RAID devices. One advantage to using software
RAID
is that you can break the mirror and backup a static copy of your database,
re-mirroring after backup is complete. The disadvantage is that software is
subject to competition with the rest of the processes running on the
computer
and can adversely impact performance.
RAID 0 (striping) works well with a Progress database and offers improved
performance by reducing head seek time.
RAID 1 (mirroring) works exceptionally well with a Progress database. Since
it
provides complete data redundancy, it is a very reliable solution that also
offers good read/write I/O performance.
**RAID 5 is not recommended by Epicor Software Corporation or by Progress.**
RAID 5 is combination of striping and parity. It offers the data
reliability
approaching that of mirroring, with the read I/O performance of striping.
However, there is a substantial performance penalty when write I/O
operations
are done. The Vantage application is extremely write I/O intensive. For
that
reason, RAID 5 IS NOT RECOMMENDED for use with a Progress database. As your
database grows in size, the performance penalty will become more apparent
over
time.
Additionally, a RAID 5 box with its own controller often has on-board write
caching that can subvert Progress write integrity algorithms, and can cause
database corruption. Make sure that any write caching is backed up by some
UPS mechanism.
As a final comment, RAID that is implemented within the NT operating system,
with the server's CPU handling write caching, can be dangerous to data
integrity; unlike dedicated RAID boxes, there isn't a reliable mechanism for
recovering write-cached data that wasn't yet written if the server goes
down.
====================================================
RAID OPTIONS - Benefits and Drawbacks.
Hard Disk Fault Tolerance
When more than one physical disk is used by the server, it is possible to
configure them in a 'stripe set' or a 'mirrored set' that is intended to
improve the speed of disk operations and to provide for tolerance in the
case of drive failure. This is a complex area, because the striping or
mirroring can be controlled by dedicated hardware, the operating system, or
even disk controller drivers. Each has merits under different circumstances.
Also, it is possible to include redundant drives that offer a certain level
of fault tolerance (useful for a mission critical system). RAID 1, RAID 5
and RAID 10 all provide for fault tolerance.
RAID Levels
==========
The RAID concept (Redundant Arrays of Inexpensive Disks) was first proposed
in 1987 by researchers Patterson, Gibson, and Katz at the University of
California at Berkeley. They proposed that the cost, performance, size
limitations, and reliability of storage subsystems could be improved by
combining several small disks into a disk "array" that is perceived as a
single disk by applications (and the operating system).
They described five ways of combining disks, numbered 1 through 5. Later,
RAID 0 was added to the list. Combination RAID levels are also possible to
an extent, RAID10 is
a combination on Raid 1 (Mirrors) and RAID 0 (Striping), in this case the
benefits of each RAID type are exploited. Each configuration, however, has
different costs, benefits, and drawbacks.
RAID levels 2, 3 and 4 are not commercially viable, and are excluded from
this document.
RAID 0
======
RAID level 0 is also called "striping". In this configuration, a group of
disks act as a single disk (a "stripe set") with data striped across all the
disks. The striping strategy is that the total storage space divided into
equal-size sections or "stripe blocks" that are allocated round-robin among
all the disks. The size of the striper block is configurable is normally a
megabyte long or longer. Technically RAID0 is not RAID at all, as there is
no redundancy.
** BENEFITS **
· Read and write performance for large sequential files is greatly
enhanced because all the disks in the stripe set can be accessed in
parallel. Read and write performance for small random access transfers is
about the same as for separate disks, but under special circumstances can
actually be worse.
· Disk activity will, on the average, be balanced across all the disks
in
the stripe set so no one disk will become a "hot spot" with much more
activity than the others. This improves performance on the average for all
users.
· Several small disks can be combined into a single large logical disk
to
provide increased storage capacity.
** DRAWBACKS**
· Reliability is extremely low. Failure of a single disk causes loss
of the entire stripe set. The mean time to failure (mttf) of the entire
stripe set is inversely proportional to the number of disks.
· Small file handling can be cumbersome if the file sizes are slightly
larger than the stripes.
RAID 1
RAID level 1 is also called "mirroring". In this configuration, a duplicate
copy of each disk is stored on a second "mirror" drive. All data are written
to both sides of the mirror. This provides redundancy and fault tolerance
because each disk is duplicated. If one fails, its mirrors can still be
accessed and no data are lost because of the failure. One can configure RAID
1 to have more than one duplicate (e.g. "triple mirroring") copy of each
disk; this is generally regarded as overkill however.
** BENEFITS **
· Reliability is very high.
· A disk failure has no impact on WRITE performance.
· If any disk fails, its mirror can continue to function and the
failed disk
can be replaced when it is convenient. Many modern storage subsystems are
designed so that failed components can be replaced while the storage
subsystem is fully operational.
· Read performance is better than single drives because data can be
read from either side of the mirror.
** DRAWBACKS**
· Cost is highest. The cost per megabyte is slightly more than doubled
(or tripled if two redundant copies are made) compared to single drives. On
the other hand, disks are increasingly inexpensive thereby offsetting this
issue to a large extent.
· There can be a very slight degradation in write performance compared
to single drives.
RAID 5
======
In RAID level 5 configurations, data are striped across several disks along
with "parity" data. The striping strategy is the same as for RAID 0
(relatively large stripe blocks). The parity data is distributed across the
drives in such a way that a data group and its parity information
are always written to different devices. This technique allows
reconstruction of all data that was present on any single drive that has
failed. Some implementations allow the parity drive to be a single drive,
others allow this parity information to be stored on the other drives in the
array.
** BENEFITS **
· READ performance can be quite good, especially where multiple disks
are RAID'ed into an array.
· Reliability will be high. If one disk fails, the remaining disks
continue to function without loss of data or availability. The failed disk
can be replaced when it is convenient.
· Cost is lower when compared with mirroring (RAID 1), a typical RAID5
array consists of three disks, giving 2/3 availability of the storage
purchased (3 x 18Gb drives would give around 36GB). ( Note: You can setup
RAID-5 arrays of up to seven drives with hotspares included. )
** DRAWBACKS**
· WRITE performance has the potential to be terrible for database
based
applications. This is because the parity data must be updated whenever a
block is written. In the worst case, writing a single database block
requires four or more (in the case of a block being split over stripes) i/o
operations. The following operations are required:
1. Read the old stripe block
2. Read the old parity data
3. Exclusive or the old data group with the parity data
4. Merge the new database block into the old data group
5. Exclusive or the new data with the parity data
6. Write the new stripe block
7. Write the new parity data
"But since the data and parity are on separate drives, they can be read in
parallel" you say. Whilst this is true, reading two disks at the same time
uses up half your disk bandwidth. As you
can see, writing can use even more than half, giving the potential for
serious WRITE performance concerns.
· Although a failed disk can be replaced, when they are,
reconstructing its
contents requires reading the entire contents of every other disk, writing
the entire contents of the failed disk, and rewriting the parity information
on all other disks. This can adversely affect overall
system performance whilst this operation takes place.
· In the case of a single Parity disk on low-cost RAID5
implementations,
this disk is often considered to be a point failure problem.
RAID 10
=======
RAID level 10 is a combination of Mirroring and Striping. This takes a
minimum of four identical drives to implement, and gives all of the
advantages of both the striping and mirroring concepts, and reduces the
disadvantages. In this configuration, a duplicate copy of each pair of
striped disks is stored on a second pair of mirror drives. All data are
written to both sides of the mirror. This provides redundancy and fault
tolerance because each disk is duplicated. If one fails, its mirrors can
still be accessed and no data are lost because of the failure. This provides
the high performance aspects of Mirrored READ's, and striped WRITE's, In a
multiple disk environment this costs no more that RAID1, and only 1/3 more
than RAID5..
** BENEFITS **
· Reliability is very high. A disk failure can have almost no impact
on
performance.
· If any disk fails, its mirror can continue to function and the
failed disk
can be replaced when it is convenient. Many modern storage subsystems are
designed so that failed components can be replaced while the storage
subsystem is fully operational (Hot Swap).
· Read and write performance for large sequential files (such as those
that databases use) is greatly enhanced because all the disks in the stripe
set can be accessed in parallel.
· Read and write performance for small random access transfers is
about twice as good as for separate disks, whilst over-all read performance
can be three or more times that of a single drive.
· Disk activity will, on the average, be balanced across all the disks
in
the stripe set so no one disk will become a "hot spot" with much more
activity than the others. This improves
performance on the average for all users.
** DRAWBACKS**
· Cost is high due to the mirroring, 50% of the hardware is redundant.
However disks are no longer expensive items.
· You cannot add "Another 18GB" without buying two 18GB disks, seems a
little odd to many people.
Note: If you are contemplating RAID 1, and wish to have multiple disks, RAID
10 will give you a performance boost over two "straight" RAID1 mirrored
implementations.
Implementation types
There are four common RAID implementations: in the operating system, in the
disk controller, via a dedicated RAID controller hardware or in a separate
subsystem. Separate disk subsystems are discussed later in this document,
but summarized here for the sake of clarity.
A RAID implementation that is part of the operating system is cheap - you
don't have to pay many dollars for it (or not much). It isn't worth much
either and should be avoided wherever possible, the additional load put on
the server, and the lack of hardware level control makes this very
undesirable.
RAID implementation in a disk controller uses less processor power and
system resources than an operating system based solution. Usually, special
device drivers are required for these disk controllers. These types of
systems are inexpensive but provide very limited fault-tolerance. Failed
disks cannot be replaced while the system is operational. This is however;
better than no RAID at all.
RAID implementation via a dedicated RAID controller card, combined with
hot-swappable disks. These are finding their way onto even entry-level
servers, and give the advantage of not having to take the machine off-line
to replace a disk; they are a very cost effective method of adding RAID to a
server.
A RAID implementation that is a separate subsystem from the computer is
best. It can have its own power supplies, battery backup, spare controllers,
hot-standby disks, and other capabilities all of which are independent of
the computer. Failure of a component in the computer should
not affect the disk subsystem and vice versa. An excellent example of this
type of system is Data General's Clariion disk array. Many other vendors
also provide these types of systems. This is discussed in more detail later
in this document.
RAID Recap
===========
RAID 0, or disk striping, is a simple disk striping strategy. Data is
written evenly across all disks in the array providing optimal read and
write performance as operations are performed across multiple disks. A major
disadvantage of RAID 0 is that it does not provide any fault tolerance and
all data is lost if one disk in the array fails. RAID 0 offers high
performance READ and WRITES, but has no fault tolerance. This type of array
is suitable for often accessed temporary files. We do not advise that RAID 0
is used to store important data. This is not technically RAID, as there is
no redundancy.
RAID 1, or mirroring, provides excellent fault tolerance as data is written
to both disks on a mirrored pair and each is a working copy of the data.
Writing to a mirrored pair takes no longer than writing to a single disk and
good disk controllers can read data almost twice as fast by reading from
both disks in parallel. However, RAID 1 implementations result in a loss of
50% of disk space that you have purchased, as data is written twice.
RAID 5, or disk stripping with parity, extends the concept of disk stripping
and uses parity to provide fault tolerance at the cost of additional reads
and writes when data blocks are updated. Data redundancy is provided by the
parity information. Although RAID 5 write performance is
substantially slower than disk striping without parity (RAID 0), striping
with parity generally offers similar READ performance to disk mirroring
(RAID 1). Disk space is needed for the parity information, rendering some
portion of disk space unusable for data. However, this loss is less than
that of RAID 1 implementations. This is a RAID level that is promoted by
many hardware venders as it is less costly that RAID1/10 whilst still
offering high levels of data security.
RAID 10 (RAID 1 + RAID 0, or RAID0+1/RAID1+0)) is also known as mirroring
with striping. This level uses a striped array of disks, which are then
mirrored to another identical set of striped disks. For example, a striped
array can be created using three disks. The striped array of disks
is then mirrored using another set of three striped disks. RAID 10 provides
the performance benefits of disk striping with the disk redundancy of
mirroring, without the performance issues associated with parity
maintenance. RAID 10 provides the highest read/write performance of any of
the RAID levels at the expense of using twice as many disks. This is advised
for high security and high performance situations.
External Disk Systems. This includes such esoteric systems as the Clariion
disk array. This is a separate subsystem that is not a physical part of the
your server. They are very fast, very secure, and often highly expensive.
They are currently the ultimate in speed and safety, and
highly recommended for enterprise level servers
With redundancy, RAID 1, 5 and 10 implementations allow for the server to
remain operational even if a drive fails. Some drive arrays also allow for
repair and rebuild of the failed drive while the server continues
uninterrupted operations, often automatically, other systems (usually less
expensive ones) demand that the server is taken off line whilst this rebuild
is accomplished. Again, these issues are complex, and cannot be thoroughly
covered by this document; they should be discussed with your hardware vendor
and implementation team.
Hardware-Based Disk Cache
=======================
Systems that contain a large READ disk cache within the controller can
dramatically improve the performance of the application server. This option
is relatively inexpensive when purchased with the overall system and
therefore is recommended, particularly for larger implementations. WRITE
caches can cause integrity issues under extremely rare, and complex
circumstances. Most caches can be configured easily enough to avoid this
rare issue by turning off the write cache.
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