5.6.2. RAID-Based Storage
One skill that a system administrator should cultivate is the ability to look at complex system configurations, and observe the different shortcomings inherent in each configuration. While this might, at first glance, seem to be a rather depressing viewpoint to take, it can be a great way to look beyond the shiny new boxes and visualize some future Saturday night with all production down due to a failure that could easily have been avoided with a bit of forethought.
With this in mind, let us use what we now know about disk-based storage and see if we can determine the ways that disk drives can cause problems. First, consider an outright hardware failure:
A disk drive with four partitions on it dies completely: what happens to the data on those partitions?
It is immediately unavailable (at least until the failing unit can be replaced, and the data restored from a recent backup).
A disk drive with a single partition on it is operating at the limits of its design due to massive I/O loads: what happens to applications that require access to the data on that partition?
The applications slow down because the disk drive cannot process reads and writes any faster.
You have a large data file that is slowly growing in size; soon it will be larger than the largest disk drive available for your system. What happens then?
The disk drive fills up, the data file stops growing, and its associated applications stop running.
Just one of these problems could cripple a data center, yet system administrators must face these kinds of issues every day. What can be done?
Fortunately, there is one technology that can address each one of these issues. The name for that technology is RAID.
5.6.2.1. Basic Concepts
RAID is an acronym standing for Redundant Array of Independent Disks[21]. As the name implies, RAID is a way for multiple disk drives to act as if they were a single disk drive.
RAID techniques were first developed by researchers at the University of California, Berkeley in the mid-1980s. At the time, there was a large gap in price between the high-performance disk drives used on the large computer installations of the day, and the smaller, slower disk drives used by the still-young personal computer industry. RAID was viewed as a method of having several less expensive disk drives fill in for one higher-priced unit.
More importantly, RAID arrays can be constructed in different ways, resulting in different characteristics depending on the final configuration. Let us look at the different configurations (known as RAID levels) in more detail.
5.6.2.1.1. RAID Levels
The Berkeley researchers originally defined five different RAID levels and numbered them "1" through "5." In time, additional RAID levels were defined by other researchers and members of the storage industry. Not all RAID levels were equally useful; some were of interest only for research purposes, and others could not be economically implemented.
In the end, there were three RAID levels that ended up seeing widespread usage:
- Level 0
- Level 1
- Level 5
The following sections discuss each of these levels in more detail.
5.6.2.1.1.1. RAID 0
The disk configuration known as RAID level 0 is a bit misleading, as this is the only RAID level that employs absolutely no redundancy. However, even though RAID 0 has no advantages from a reliability standpoint, it does have other benefits.
A RAID 0 array consists of two or more disk drives. The available storage capacity on each drive is divided into chunks, which represent some multiple of the drives' native block size. Data written to the array is be written, chunk by chunk, to each drive in the array. The chunks can be thought of as forming stripes across each drive in the array; hence the other term for RAID 0: striping.
For example, with a two-drive array and a 4KB chunk size, writing 12KB of data to the array would result in the data being written in three 4KB chunks to the following drives:
- The first 4KB would be written to the first drive, into the first chunk
- The second 4KB would be written to the second drive, into the first chunk
- The last 4KB would be written to the first drive, into the second chunk
Compared to a single disk drive, the advantages to RAID 0 include:
- Larger total size -- RAID 0 arrays can be constructed that are larger than a single disk drive, making it easier to store larger data files
- Better read/write performance -- The I/O load on a RAID 0 array is spread evenly among all the drives in the array (Assuming all the I/O is not concentrated on a single chunk)
- No wasted space -- All available storage on all drives in the array are available for data storage
Compared to a single disk drive, RAID 0 has the following disadvantage:
- Less reliability -- Every drive in a RAID 0 array must be operative for the array to be available; a single drive failure in an N-drive RAID 0 array results in the removal of 1/Nth of all the data, rendering the array useless
Note
If you have trouble keeping the different RAID levels straight, just remember that RAID 0 has zero percent redundancy.
5.6.2.1.1.2. RAID 1
RAID 1 uses two (although some implementations support more) identical disk drives. All data is written to both drives, making them mirror images of each other. That is why RAID 1 is often known as mirroring.
Whenever data is written to a RAID 1 array, two physical writes must take place: one to the first drive, and one to the second drive. Reading data, on the other hand, only needs to take place once and either drive in the array can be used.
Compared to a single disk drive, a RAID 1 array has the following advantages:
- Improved redundancy -- Even if one drive in the array were to fail, the data would still be accessible
- Improved read performance -- With both drives operational, reads can be evenly split between them, reducing per-drive I/O loads
When compared to a single disk drive, a RAID 1 array has some disadvantages:
- Maximum array size is limited to the largest single drive available.
- Reduced write performance -- Because both drives must be kept up-to-date, all write I/Os must be performed by both drives, slowing the overall process of writing data to the array
- Reduced cost efficiency -- With one entire drive dedicated to redundancy, the cost of a RAID 1 array is at least double that of a single drive
Note
If you have trouble keeping the different RAID levels straight, just remember that RAID 1 has one hundred percent redundancy.
5.6.2.1.1.3. RAID 5
RAID 5 attempts to combine the benefits of RAID 0 and RAID 1, while minimizing their respective disadvantages.
Like RAID 0, a RAID 5 array consists of multiple disk drives, each divided into chunks. This allows a RAID 5 array to be larger than any single drive. Like a RAID 1 array, a RAID 5 array uses some disk space in a redundant fashion, improving reliability.
However, the way RAID 5 works is unlike either RAID 0 or 1.
A RAID 5 array must consist of at least three identically-sized disk drives (although more drives may be used). Each drive is divided into chunks and data is written to the chunks in order. However, not every chunk is dedicated to data storage as it is in RAID 0. Instead, in an array with n disk drives in it, every nth chunk is dedicated to parity.
Chunks containing parity make it possible to recover data should one of the drives in the array fail. The parity in chunk x is calculated by mathematically combining the data from each chunk x stored on all the other drives in the array. If the data in a chunk is updated, the corresponding parity chunk must be recalculated and updated as well.
This also means that every time data is written to the array, at least two drives are written to: the drive holding the data, and the drive containing the parity chunk.
One key point to keep in mind is that the parity chunks are not concentrated on any one drive in the array. Instead, they are spread evenly across all the drives. Even though dedicating a specific drive to contain nothing but parity is possible (in fact, this configuration is known as RAID level 4), the constant updating of parity as data is written to the array would mean that the parity drive could become a performance bottleneck. By spreading the parity information evenly throughout the array, this impact is reduced.
However, it is important to keep in mind the impact of parity on the overall storage capacity of the array. Even though the parity information is spread evenly across all the drives in the array, the amount of available storage is reduced by the size of one drive.
Compared to a single drive, a RAID 5 array has the following advantages:
- Improved redundancy -- If one drive in the array fails, the parity information can be used to reconstruct the missing data chunks, all while keeping the array available for use[22]
- Improved read performance -- Due to the RAID 0-like way data is divided between drives in the array, read I/O activity is spread evenly between all the drives
- Reasonably good cost efficiency -- For a RAID 5 array of n drives, only 1/nth of the total available storage is dedicated to redundancy
Compared to a single drive, a RAID 5 array has the following disadvantage:
- Reduced write performance -- Because each write to the array results in at least two writes to the physical drives (one write for the data and one for the parity), write performance is worse than a single drive[23]
5.6.2.1.1.4. Nested RAID Levels
As should be obvious from the discussion of the various RAID levels, each level has specific strengths and weaknesses. It was not long after RAID-based storage began to be deployed that people began to wonder whether different RAID levels could somehow be combined, producing arrays with all of the strengths and none of the weaknesses of the original levels.
For example, what if the disk drives in a RAID 0 array were themselves actually RAID 1 arrays? This would give the advantages of RAID 0's speed, with the reliability of RAID 1.
This is just the kind of thing that can be done. Here are the most commonly-nested RAID levels:
- RAID 1+0
- RAID 5+0
- RAID 5+1
Because nested RAID is used in more specialized environments, we will not go into greater detail here. However, there are two points to keep in mind when thinking about nested RAID:
- Order matters -- The order in which RAID levels are nested can have a large impact on reliability. In other words, RAID 1+0 and RAID 0+1 are not the same.
- Costs can be high -- If there is any disadvantage common to all nested RAID implementations, it is one of cost; for example, the smallest possible RAID 5+1 array consists of six disk drives (and even more drives are required for larger arrays).
Now that we have explored the concepts behind RAID, let us see how RAID can be implemented.
5.6.2.1.2. RAID Implementations
It is obvious from the previous sections that RAID requires additional "intelligence" over and above the usual disk I/O processing for individual drives. At the very least, the following tasks must be performed:
- Dividing incoming I/O requests to the individual disks in the array
- For RAID 5, calculating parity and writing it to the appropriate drive in the array
- Monitoring the individual disks in the array and taking the appropriate action should one fail
- Controlling the rebuilding of an individual disk in the array, when that disk has been replaced or repaired
- Providing a means to allow administrators to maintain the array (removing and adding drives, initiating and halting rebuilds, etc.)
There are two major methods that may be used to accomplish these tasks. The next two sections describe them in more detail.
5.6.2.1.2.1. Hardware RAID
A hardware RAID implementation usually takes the form of a specialized disk controller card. The card performs all RAID-related functions and directly controls the individual drives in the arrays attached to it. With the proper driver, the arrays managed by a hardware RAID card appear to the host operating system just as if they were regular disk drives.
Most RAID controller cards work with SCSI drives, although there are some ATA-based RAID controllers as well. In any case, the administrative interface is usually implemented in one of three ways:
- Specialized utility programs that run as applications under the host operating system, presenting a software interface to the controller card
- An on-board interface using a serial port that is accessed using a terminal emulator
- A BIOS-like interface that is only accessible during the system's power-up testing
Some RAID controllers have more than one type of administrative interface available. For obvious reasons, a software interface provides the most flexibility, as it allows administrative functions while the operating system is running. However, if you are booting an operating system from a RAID controller, an interface that does not require a running operating system is a requirement.
Because there are so many different RAID controller cards on the market, it is impossible to go into further detail here. The best course of action is to read the manufacturer's documentation for more information.
5.6.2.1.2.2. Software RAID
Software RAID is RAID implemented as kernel- or driver-level software for a particular operating system. As such, it provides more flexibility in terms of hardware support -- as long as the hardware is supported by the operating system, RAID arrays can be configured and deployed. This can dramatically reduce the cost of deploying RAID by eliminating the need for expensive, specialized RAID hardware.
Often the excess CPU power available for software RAID parity calculations greatly exceeds the processing power present on a RAID controller card. Therefore, some software RAID implementations actually have the capability for higher performance than hardware RAID implementations.
However, software RAID does have limitations not present in hardware RAID. The most important one to consider is support for booting from a software RAID array. In most cases, only RAID 1 arrays can be used for booting, as the computer's BIOS is not RAID-aware. Since a single drive from a RAID 1 array is indistinguishable from a non-RAID boot device, the BIOS can successfully start the boot process; the operating system can then change over to software RAID operation once it has gained control of the system.
[21]
When early RAID research began, the acronym stood for Redundant Array of Inexpensive Disks, but over time the "standalone" disks that RAID was intended to supplant became cheaper and cheaper, rendering the price comparison meaningless.
[22]
I/O performance is reduced while operating with one drive unavailable, due to the overhead involved in reconstructing the missing data.
[23]
There is also an impact from the parity calculations required for each write. However, depending on the specific RAID 5 implementation (specifically, where in the system the parity calculations are performed), this impact can range from sizable to nearly nonexistent.
