Timothy
Parker Consulting Incorporated
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RAID:
Saving Your Data RAID
-- Redundant Array of Inexpensive Drives (or Disks, depending on whom you ask)
– has been around for years, originally fueled by the need to ensure high data
integrity and availability. The
basic premise of storing data on more than one drive to make sure nothing gets
lost, and the ability to recover from failed disk drives without shutting down
the system has made RAID a staple for many larger systems, mostly UNIX-based,
but also gradually easing into the DOS and Windows worlds as those operating
systems became the drivers of critical servers. The concepts behind RAID are to
achieve a point where the parameter Mean Time to Data Availability (MTDA) is
minimal and Mean Time to Data Loss (MTDL) is infinitely large.
In other words, you never have to wait for your system to recover from a
disk problem, and you never will lose information.
Windows
NT is ideally suited as a mission-critical operating system because of its high
reliability and strong security. RAID
subsystems have been available for Windows NT from the earliest versions, and
Windows NT 4 extends the support to provide built-in RAID capabilities and a
reasonably long list of supported RAID controllers. In this article we look at what RAID is, how it works, how
you use it, and how to get it installed on your systems. We’ll try and show you why RAID may be important to you and
why you should consider implementing it. But
first, let’s figure out what RAID is. The
RAID Levels RAID
methodology was first proposed at the University of California at Berkeley,
although it was not envisioned in quite the same way we use RAID today. RAID was
defined as a memory architecture using a subsystem of two or more hard disk
drives treated as a single, larger logical drive. The purpose of this proposed architecture was to take
advantage of data redundancy inherent in the multiple drive design, as well as
to capitalize on the lower costs of smaller drives. (When RAID was first proposed it was considerably cheaper to
buy five 200MB hard drives than one larger 1GB drive, for example.
This not true any more, so the current focus for RAID is data integrity
and reliability instead of cost saving.) Originally
there were six levels of RAID (called RAID 0 through RAID 5), and a few more
have been added lately to combine features of other levels.
Although RAID 6 follows the general numbering process, most of the new
levels break with the number sequence and create numbers of their own, usually
for marketing reasons. Not all of
the RAID levels are commercially available, and some are
supported by only a few products. An
example is RAID 6, which very few products implement.
Only RAID levels 0, 1, and 5 are supported by Windows NT 4 without
additional hardware and software from RAID vendors. Those three levels, though, can be supported by a variety of
hard disk and controller combinations. RAID
0 is the base level of the RAID technology.
RAID 0 stripes data across all drives.
Striping means that all available hard drives are combined into a single
large virtual filesystem, with the blocks of the filesystem arrayed so that they
are spread evenly across all the drives. For
example, if you have three 500MB hard drives, RAID 0 provides for a 1.5GB
virtual hard drive. When you store
files, they are written across all three drives. When a large file, such as a
100MB multimedia presentation, is saved to the virtual drive, a part of it may
be written to the first drive, the next chunk to the second, more to the third,
and perhaps more wrapping back to the first drive to start the sequence again.
The exact manner in which the chunks of data move from physical drive to
physical drive depends on the way the virtual drive has been set up, which
includes considering drive capacity and the way in which blocks are allocated on
each drive. No
parity control is used with RAID 0, which is the major problem with this level.
There is no data redundancy at all with RAID 0, either, as the data is
only written once. RAID 0 does have
a number of advantages, through. Most
important for many users is that striping across three or more drives does
provide some increase in performance through load-balancing.
This is especially noticeable with large files.
For example, to return to our 100MB file save, if 33MB was written to
each of the three hard drives in our RAID 0 system, all three drives can save
data at the same time. The total
save time for the file is theoretically one third that of writing the full file
on one drive. Reading data is
similar, in that the three thirds of the large file can be read simultaneously
and combined by the RAID software, again theoretically reducing the read time to
one third. In practice, the savings
are not that dramatic due to factors such as drive latency and head seek times,
but RAID 0 arrays of three or more drives can noticeably affect read and write
performance. On the author’s RAID
0 system comprising four drives, read and write performance is less than half
that of a single large drive. RAID
0 has a side advantage that many users will like in that it allows drives of
differing sizes and layouts to be used. For
example, if you have a couple of drives of only a few hundred megabytes capacity
that have been replaced in a server by high capacity drives, you can use the
smaller units in a RAID 0 array with no penalty.
Windows NT 4 supports RAID 0 setups of three to thirty-two disks.
RAID
1 is known as disk mirroring or disk shadowing. With RAID 1, each hard drive on the system has a duplicate
drive which contains an exact copy of the first drive’s contents.
Since every bit written to the filesystem is duplicated, there is data
redundancy with RAID 1. If one drive in
the RAID 1 array fails or develops a problem of any kind (such as a bad sector),
the mirror drive can take over and maintain all normal filesystem operations
while the faulty drive is diagnosed and fixed.
Many RAID 1 disk controllers have software routines that will
automatically take a faulty drive off-line, run diagnostics on it and, if
possible, reformat the drive and copy all data back from the mirror image all
while the filesystem proceeds as if nothing had happened.
Users are usually unaware of faults with RAID 1 controllers except for
alert messages that can be triggered. One
big disadvantage of RAID 1 is the use of disks. If you have two 2GB drives, you can only have a total
filesystem of 2GB (the other 2GB is mirrored).
You’re only getting half the disk space you’re paying for, but you do
have fully redundant drives. In
case of catastrophic failure of a drive, a controller, or a motherboard, you can
remove a mirror drive and boot on another controller or server.
RAID
1 offers an increase in read performance in most implementations, as the
controller card allows both of the drives (primary and mirror) to be read at the
same time, resulting in a faster read operation. Write operations are not faster, though, as there are two
drives that must be written. In
many RAID 1 systems that do not use separate drive controllers for the primary
and mirror drives, writing can even slow down as two complete write operations
must be performed in sequence. Testing
on one of the author’s systems using a single SCSI controller card shows that
the speed increase of the read operation is more than offset by write delays,
with a net effect of a slight degradation in system performance.
With two controller cards (one for each drive), performance increases
slightly due to the read advantages, but only when the two controllers are able
to cooperate with each other. Implementations
of RAID 1 usually require two drives of similar size. If you use a 1.5GB and a 2GB drive, for example, the extra
0.5GB on the second drive is wasted. Some
controllers will allow you to combine drives of different sizes, with the extra
used for non-mirrored partitions. Windows
NT 4 supports RAID 1 with two or more drives.
RAID
2 tries to overcome the speed limitations of RAID 1 by using a technique called
Hamming codes. Hamming showed that data can be organized in such a manner that
if an error develops affects only
one bit in each byte, the error can be detected and corrected without an
inordinate amount of system overhead. In
the classic example of RAID 2, nine disk drives can be used in an array which
has the first bit of every byte written on the first drive, the second bit of
the byte on the second drive, and so on. The
ninth drive has an error correction bit. A fault affecting one drive’s read of a bit will be
detected by the correction code, and fixed with a simple algorithm.
The same method can be used with less than nine drives, writing more than
one bit per drive but then increasing the chances that a damaged hard drive will
lead to unrecoverable errors. All
drives in a RAID 2 array read and write data in parallel (as with RAID 0), so
throughput is faster. In theory,
the amount of throughput increase by the same factor as the number of drives
involved, so a nine-drive array will result in read and write operations taking
one eighth the amount of time (ignoring the error-correction drive).
In practice, RAID 2 systems don’t come near this theoretical figure,
but they do show a noticeably increase in performance.
The major problem with RAID 2 is that it requires large disk arrays,
which are seldom practical for small server systems.
RAID 2 is not inherently supported by Windows NT 4, but some commercial
implementations are adaptable. The
sheer size and cost of a RAID 2 subsystem generally rule them out in favor of
other RAID systems. RAID
3 tries to combine the techniques of RAID 0 and RAID 2 into a more
reasonably-priced system. With RAID
3, two or more drives hold data while an extra drive holds error correction.
Data is interleaved across the data drives so that the first byte is on
the first drive, the second byte on the second, and so on.
When all the data drives are used, the storage loops back to the first
data drive. The number of drives affects which bits are stored where, so
in a two-drive array drive one will have the odd-numbered bits and drive two
will have the even-numbered bits. The
error correction drive holds a bit-sensitive value for each sector on the data
drives (it actually uses the same algorithm as that employed with parity RAM).
With eight data drives, the system is very similar to RAID 2 except the
error correction algorithm is different. As
you would expect, RAID 3 offers performance increase because of simultaneous
reads and writes. RAID 3 is often found on larger minicomputer systems where
huge files must be handled and the performance advantage RAID 3 offers is
noticeable. As with RAID 2, though,
the cost of setting up a RAID 3 subsystem can be prohibitive, and there are
better methods to achieve the same result.
RAID 3 is not supported by Windows NT 4, although some commercial systems
are available. RAID
4 tries to solve the primary problem that occurs with RAID 2 and RAID 3, the
need for sequential disk I/Os and the need for large block sizes on the drives.
With RAID 4, larger blocks of data (such as a sector) are written to the
first drive, the next to the second, and so on, instead of bits as with RAID 2
and RAID 3. A single error correction drive is involved. In
case of a data disk error, missing data can be reconstructed from the error
correction codes. Because
of the use of larger disk blocks, small file reads and writes can be performed
simultaneously with RAID 4, spread over the entire disk array.
Optimization algorithms can adjust the layout of data across the drives
so that frequently read files are spread out for fastest retrieval.
As with RAID 2 and RAID 3, RAID 4 is not supported by Windows NT. RAID
5 addresses a significant flaw apparent with RAID 4. Since RAID 4 uses a dedicated error correction drive, write
limitations are inherent as codes are written sequentially to the single error
correction drive. Although this is
not a problem for most operating systems, for those involved in high data
throughput (such as transaction processing) this single error-correction write
process can become a bottleneck. RAID
5 addresses this problem by removing the dedicated error correction drive and
spreading the error correction data across all the hard drives in the array.
Each drive in the array then holds data and error correction information.
RAID 5 is often called striping with parity, since the data is striped as
with RAID 0 and a parity code is also written to allow recovery of corrupted
data. On
heavily loaded systems, RAID 5 doesn’t improve read times over RAID 4 but
write times are better because they can be performed in parallel.
Tests of heavily loaded UNIX workstations running RAID 5 show write times
halved by this technique over an equivalent RAID 4 subsystem.
In theory, RAID 5 allows all drives to read and write in parallel so as
the number of drives in the array increases, transfer rates drop by the inverse
(so if four drives are used, read and write operations are a quarter of the
speeds found with one drive). The
primary problem with RAID 5 is that when a drive encounters an error or fails,
the recovery algorithm required to rebuild lost data can become significant and
load the operating system down. RAID
5 is supported by Windows NT 4 and most RAID vendors as it is a good compromise
between data integrity, speed, and cost. RAID
5 has better performance that RAID 1 (mirroring) and there is a performance
increase. RAID 5 usually requires
at least three drives, with more drives preferable.
The overhead RAID 5 imposes on RAM can be significant, too, so Microsoft
recommends at least 16MB RAM when RAID 5 is used.
As with RAID 1, though, drives of disparate capacities may result in a
lot of unused disk space, as most RAID 5 systems use the smallest drive capacity
in the array for all the RAID 5 drives. Extra
disk space can be used for un-striped partitions, but these are not protected by
the RAID system. As
mentioned earlier, there are a few new RAID levels and systems that have
appeared over the years, although none have gained a significant following.
Support for Windows NT 4 varies. RAID
10 was introduced by RAID vendor ECCS, offering mirrored pairs of drives with
block striping between the drives. RAID 10 is essentially a combination of RAID
1 and RAID 0. RAID 10 offers very good scalability with the ability to add new
drives as needed, as well as offering good data redundancy, but it is useful to
remember that RAID 10 is really just a combination of RAID 0 and RAID 1 with a
few bells and whistles. RAID 10
subsystems tend to be expensive, usually enough to price them out of the
realistic range for Windows NT servers. Another
highly-touted system, RAID 53, was introduced by RAID vendor Hi-DATA.
RAID 53 is intended to combine the reliability of RAID 3 with the lower
cost of RAID 5. Hi-DATA’s system
uses a RAID 3 array for each drive in a RAID 5 model.
As with ECCS’ RAID 10, the cost of a RAID 53 subsystem becomes higher
than is reasonable for an average server. RAID
enjoyed a surge in popularity in the late eighties as desktop UNIX systems
became attractive in terms of performance and reliability.
Many companies saw a chance to dump dedicated minicomputers and use a PC
with an Intel UNIX such as SCO UNIX. The
ability to use RAID subsystems on these servers made data integrity and
reliability just as good as dedicated minicomputer drive arrays, for a lot less
money. Several companies sprang up to offer RAID products to this
growing desktop PC market, and while many have faded away, the strongest have
continued to refine their Intel platform products, including ports to Windows
NT. When
they first appeared for desktop PC machines, RAID systems were composed of ESDI
and SCSI drives. ESDI, of course,
virtually disappeared within a couple of years, but SCSI has remained popular
and is still the best choice in terms of support and performance for RAID.
IDE and EIDE RAID subsystems are also appearing now, although EIDE
limitations of four drives (only two with IDE) limits their applicability to
only mirroring (RAID 1 and, to a lesser extent, RAID 5).
According to the RAID vendors, SCSI subsystems represent over ninety-five
percent of the RAID market. SCSI’s
high throughput and features like hot-swappable drives (where a drive can be
removed without powering down the system) make it very attractive over all other
drive formats. Also, the
development of Ultra SCSI with throughputs to 40Mbps has made SCSI subsystems
almost so fast the host server processors have become the limiting factor. Implementing
RAID on your Windows NT system Windows
NT 4 supports only RAID 0, 1, and 5 natively, and while there are a few others
such as RAID 3 available, we’ll focus on the three levels Microsoft deems most
important. In general, RAID can be
implemented either in software or in hardware (apart from the disk array, of
course). The specific RAID version
you want to employ determines which method is best but most operating systems
rely on software primarily. Windows NT is not exception. RAID 5, for example, is usually implemented only in software
although a multi-channel SCSI hard disk controller can be used to increased
performance. RAID
1 is almost always software-based
and since the drivers are so small they offer virtually no drain on the system.
Hardware-based implementation of
RAID 1 usually requires dual drive adapters which can actually slow the system
down more than the RAID 1 software implementation does (primarily because of
dual DMA calls).. Is
there a performance issue that you must deal with in deciding which RAID level
to employ on your system? To
develop a concrete answer, we constructed a test suite based on a Windows NT 4
server installed on a 150Mhz Pentium Pro with 128MB RAM, running Lotus Notes to
provide a variable load of read and write operations. We wanted to use the same controller card throughout the
tests to eliminate SCSI controller card performance variations, so we chose the
DPT SmartRAID IV Ultra controller. The
SmartRAID IV Ultra is a PCI SCSI controller with three channels (we only used)
and 4MB cache RAM. The DPT system
supports RAID levels 0, 1, and 5 and includes software add-ons for Windows NT 4.
All hard drives used in the test were off-the-shelf 2GB Ultra SCSI units
from the same manufacturer. We
started with a single 2GB hard drive and measured performance times with light,
medium, and heavy loads. We then
set up RAID 1 (disk mirroring) with a second 2GB drive, first with both drives
on the same controller channel and then with each drive on a separate channel.
This was followed by a RAID 0 test with four drives on one controller
channel. Finally, we installed RAID
5 with three 2GB drives on two different Ultra SCSI channels.
The performance numbers were normalized to the non-RAID values and are
shown in Table 1. Since significant
difference in read and write operation times were recorded, both numbers are
reported. All values have been
rounded for clarity. As
you can see, all three RAID implementation had a noticeably improvement in
performance for read operations, but write performance differed noticeably.
Also bear in mind that some of the performance improvements are due to
the 4MB cache on our DPT controller card. RAID
0 performance increases as drives are added to the system, and a quick test with
only two drives showed our performance numbers dropped to almost the non-RAID
numbers (we tested RAID 0 with four drives). RAID 5 read operations are faster
than either RAID level (due to the higher number of drives) but write operations
are not as fast as RAID 0. RAID 1
read operations are fast, as you would expect from parallel reads, but writes
are not improved noticeably (the slight performance increase in the table is
probably due to the controller cache). Fault
tolerance is the big separator when it comes to choosing a RAID level.
With RAID 0 you have no redundancy: a drive failure means loss of data.
RAID 1 allows automatic switch-over to a mirror unless both drives fail
at the same time (unlikely except for surge problems).
In general, though, no data is lost with RAID 1.
RAID 5 is similar, in that a drive failure means no lost data, except
when two drives fail. When a RAID 5 drive fails, though, performance is degraded
due to the algorithmic reconstruction of lost data. RAID
5 is the RAID version most often recommended, especially for business systems
where data availability is critical. RAID
5 is the most successful RAID version usually implemented, and with large disk
subsystems RAID 5 approaches both the MTDA and MTDL goals mentioned at the
beginning of this article. To
implement one of the RAID levels you need to decide whether you will purchase a
complete RAID subsystem, or build your own from controllers and hard disks.
The former is the easiest, but usually more expensive.
Several vendors offer RAID subsystems that require only the installation
of a card in your PC, some installation software, then a cable run from the
controller card to the disk subsystem to provides the entire RAID setup for you.
The main vendors of these plug-and-go RAID subsystems which have been
certified by Microsoft are Micropolis (Radion subsystems), DEC and Hitachi (both
of whom offers several subsystems), Storage Solutions (the STAC-a-ray
subsystem), and Compaq (their Ragingsrv RAID system).
For
high-performance high-reliability servers, the plug-and-go subsystems are often
the best choice. A good example is
Boxhill System’s Fibre Box is a distributed RAID subsystem for Windows NT that
allows all disk subsystem processing to take place away from the server’s
motherboard, and as the name suggests the subsystem uses fibre optics to provide
data transfer rates to a whopping 200Mbps. With disk capacities of over 72GB per box, and a maximum size
over a terabyte, this type of subsystem is for the serious server. Alternatively,
you can rely on Windows NT to do most of the work with a standard SCSI card
(better is a SCSI card with RAID support provided on-card) and a number of
drives. To install RAID on your machine, all you need is the drives, a
controller that is supported by Windows NT, and the patience to set it up
correctly. This is, luckily, quite
easy. The choice of the
SCSI controller card is often personal, and there are many available that
support Windows NT RAID, including cards from Adaptec, BusLogic, DPT, DEC,
Future Domain, Mylex, NCR, Qlogic, Trantor, and UltraStor. Some
of the controller cards, such as the DTP SmartRAID IV used in our performance
tests, are intended specifically for RAID and provide add-on software to enhance
the RAID subsystem’s performance. As
an example, DPT includes Storage Manager as a GUI-based disk array control
package, which makes setting up and using the RAID system almost trivial.
The software automatically inventories all devices attached to the
controller, provides the best options for a RAID subsystem, and lets you monitor
performance at any time. Total time
for installation on our test server, from opening the box to completing the
Windows NT software installation was just under one hour, most of which was used
in reading the Windows NT CD-ROM. Is
RAID worth the effort? If you value
your data, without a doubt. A
simple two-drive RAID 1 implementation is inexpensive compared to data loss,
involving only the purchase of a second hard drive.
The entire package can be added to existing systems for a few hundred
dollars, depending on the drive. Since
the software for mirroring is part of the Windows NT Disk Administrator package,
there’s nothing special to installation and configuration.
RAID 0 and RAID 5 are more expensive to implement, but do offer more
security and faster throughput. A
four-disk RAID 5 subsystem can be assembled for a couple of thousand dollars,
for example. The magic of RAID 0
and RAID 5 is usually ignored until the first drive failure, and then when you
see RAID do it’s magic and recover gracefully, you’ll wonder why you
didn’t buy RAID earlier.
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tparker@tpci.com with
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