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Windows Server 2003 : Planning Fault Tolerance and Avoidance (part 2) - Disk Arrays

8/12/2011 5:48:32 PM

Disk Arrays

The most common hardware malfunction is probably a hard disk failure. Even though hard disks have become more reliable over time, they are still subject to failure, especially during their first month or so of use. They are also vulnerable to both catastrophic and degenerative failures caused by power problems. Fortunately, disk arrays have become the norm for most servers, and good fault-tolerant RAID systems are available in Windows Server 2003 and RAID-specific hardware supported by Windows Server 2003. The choice of software or hardware RAID, and the particulars of how you configure your RAID system, can significantly affect the cost of your servers. To make an informed choice for your environment and needs, you must understand the trade-offs and the differences in fault tolerance, speed, configurability, and so on.

Hardware vs. Software

RAID can be implemented at the hardware level, using RAID controllers, or at the software level, either by the operating system or by a third-party add-on. Windows Server 2003 supports both hardware RAID and its own software RAID.

Hardware RAID implementations require specialized controllers and cost significantly more than an equal level of software RAID. However, for that extra price, you get a faster, more flexible, and more fault-tolerant RAID. When compared to the software RAID provided in Windows Server 2003, a good hardware RAID controller supports more levels of RAID, on-the-fly reconfiguration of the arrays, hot-swap and hot-spare drives, and dedicated caching of both reads and writes.

Implementing software RAID in Windows Server 2003 requires that you first convert your disks to dynamic disks. That means your disks will no longer be locally available to other operating systems, although this really shouldn’t be a problem in a production environment since dual-boot is rarely used there. However, you should consider carefully whether you want to convert your boot disk to a dynamic disk. Dynamic disks can be more difficult to access if a problem occurs, and the Windows Server 2003 setup and installation program provides only limited support. For maximum fault tolerance, we recommend using hardware mirroring (RAID 1) on your boot drive; if you do use software mirroring, make sure that you create the required fault-tolerant boot floppy disk and test it thoroughly before you need it.

RAID Levels for Fault Tolerance

Except for level 0, RAID is a mechanism for storing sufficient information on a group of hard disks such that even if one hard disk in the group fails, no information is lost. Some RAID arrangements go even further, providing protection in the event of multiple hard disk failures. The more common levels of RAID and their appropriateness in a fault-tolerant environment are shown in Table 1.

Table 1. RAID levels and their fault tolerance
LevelNumber of Disks[*]SpeedFault ToleranceDescription
0N+++- - -Striping alone. Not fault-tolerant—it actually increases your risk of failure—but does provide for the fastest read and write performance.
12N+++Mirror or duplex. Slightly faster read than single disk, but no gain during write operations. Failure of any single disk causes no loss in data and minimal performance hit.
3N+1+++Byte-level parity. Data is striped across multiple drives at the byte level with the parity information written to a single dedicated drive. Reads are much faster than with a single disk, but writes operate slightly slower than a single disk because parity information must be generated and written to a single disk. Failure of any single disk causes no loss of data but can cause a significant loss of performance.
4N+1+++Block-level parity with a dedicated parity disk. Similar to RAID-3 except that data is striped at the block level.
5N+1+++Interleaved block-level parity. Parity information is distributed across all drives. Reads are much faster than a single disk but writes are significantly slower. Failure of any single disk provides no loss of data but results in a major reduction in performance.
0+1 and 102N+++++Striped mirrored disks or mirrored striped disks. Data is striped across multiple mirrored disks or multiple striped disks are mirrored. Failure of any one disk causes no data loss and no speed loss. Failure of a second disk could result in data loss. Faster than a single disk for both reads and writes.
OtherVaries++++++Array of RAID arrays. Different hardware vendors have different proprietary names for this RAID concept. Excellent read and write performance. Failure of any one disk results in no loss of performance and continued redundancy.

[*] In the Number of Disks column, N refers to the number of hard disks required to hold the original copy of the data. The plus and minus symbols show relative improvement or deterioration compared to a system using no version of RAID. The scale peaks at three symbols.

When choosing the RAID level to use for a given application or server, consider the following factors:

  • Intended use Will this application be primarily read intensive, such as file serving, or will it be predominantly write intensive, such as a transactional database?

  • Fault tolerance How critical is this data, and how much can you afford to lose?

  • Availability Does this server or application need to be available at all times, or can you afford to be able to reboot it or otherwise take it offline for brief periods?

  • Performance Is this application or server heavily used, with large amounts of data being transferred to and from it, or is this server or application less I/O intensive?

  • Cost Are you on a tight budget for this server or application, or is the cost of data loss or unavailability the primary driving factor?

You need to evaluate each of these factors when you decide which type of RAID to use for a server or portion of a server. No single answer fits all cases, but the final answer requires you to carefully weigh each of these factors and balance them against your situation and your needs. The following sections take a closer look at each factor and how it weighs in the overall decision-making process.

Intended Use

The intended use, and the kind of disk access associated with that use, plays an important role in determining the best RAID level for your application. Think about how write intensive the application is and whether the manner in which the application uses the data is more sequential or random. Is your application a three-square-meals-a-day kind of application, with relatively large chunks of data being read or written at the same time, or is it more of a grazer or nibbler, reading and writing little bits of data from all sorts of different places?

If your application is relatively write intensive, you’ll want to avoid software RAID if possible and avoid RAID-5 if other considerations don’t force you to use it. With RAID-5, any application that requires more than 50 percent writes to reads is likely to be at least somewhat slower, if not much slower, than it would be on a single disk. You can mitigate this to some extent by using more but smaller drives in your array and by using a hardware controller with a large cache to offload the parity processing as much as possible. RAID-1, in either a mirror or duplex configuration, provides a high degree of fault tolerance with no significant penalty during write operations—a good choice for the Windows Server 2003 system disk.


Mirroring won’t protect you from data corruption caused by a catastrophic power interruption to a write cached system disk. Disabling write caching on boot and system volumes can is highly recommended if your system isn’t protected by a UPS. And no UPS can protect you from tripping over the power cord. A good, battery-backed cache, however, will protect you even then.

If your application is primarily read intensive, and the data is stored and referenced sequentially, RAID-3 or RAID-4 might be a good choice. Because the data is striped across many drives, you have parallel access to it, improving your throughput. And because the parity information is stored on a single drive rather than dispersed across the array, sequential read operations don’t have to skip over the parity information and are therefore faster. However, write operations are substantially slower, and the single parity drive can become an I/O bottleneck during write operations.

If your application is primarily read-intensive and not necessarily sequential, RAID-5 is an obvious choice. It provides a good balance of speed and fault tolerance, and the cost is substantially lower than the cost of RAID-1. Disk accesses are evenly distributed across multiple drives, and no one drive has the potential to be an I/O bottleneck. However, writes require calculation of the parity information and the extra write of that parity, slowing write operations down significantly.

If your application provides other mechanisms for data recovery or uses large amounts of temporary storage, which doesn’t require fault tolerance, a simple RAID-0, with no fault tolerance but fast reads and writes, is a possibility.

Fault Tolerance

Carefully examine the fault tolerance of each of the possible RAID choices for your intended use. All RAID levels except RAID-0 provide some degree of fault tolerance, but the effect of a failure and the ability to recover from subsequent failures can be different.

If a drive in a RAID-1 mirror or duplex array fails, a full, complete, exact copy of the data remains. Access to your data or application is unimpeded, and performance degradation is minimal, although you do lose the benefit gained on read operations of being able to read from either disk. Until the failed disk is replaced, however, you have no fault tolerance on the remaining disk. Once you replace the failed disk, overall performance is significantly reduced while the new disk is initialized and the mirror is rebuilt.

In a RAID-3 or RAID-4 array, if one of the data disks fails, a significant performance degradation occurs because the missing data needs to be reconstructed from the parity information. Also, you’ll have no fault tolerance until the failed disk is replaced. If it is the parity disk that fails, you’ll have no fault tolerance until it is replaced, but also no performance degradation. Once you replace the failed disk, overall performance is significantly reduced while the new disk is initialized and the parity information or data is rebuilt.

In a RAID-5 array, the loss of any disk results in a significant performance degradation, and your fault tolerance will be gone until you replace the failed disk. Once you replace the disk, you won’t return to fault tolerance until the entire array has a chance to rebuild itself, and performance is seriously degraded during the rebuild process.

RAID systems that are arrays of arrays can provide for multiple failure tolerance. These arrays provide for multiple levels of redundancy and are appropriate for mission-critical applications that must be able to withstand the failure of more than one drive in an array.


All levels of RAID, except RAID-0, provide higher availability than a single drive. However, if availability is expanded to also include the overall performance level during failure mode, some RAID levels provide definite advantages over others. Specifically, RAID-1, mirroring/duplexing, provides enhanced availability when compared to RAID levels 3, 4, and 5 during failure mode. There is minimal performance degradation when compared to a single disk if one half of a mirror fails, whereas a RAID-5 array has substantially compromised performance until the failed disk is replaced and the array is rebuilt.

In addition, RAID systems that are based on an array of arrays can provide higher availability than RAID levels 1 through 5. Running on multiple controllers, these arrays are able to tolerate the failure of more than one disk and the failure of one of the controllers, providing protection against the single point of failure inherent in any single-controller arrangement. RAID-1 that uses duplexed disks running on different controllers—as opposed to RAID-1 that uses mirroring on the same controller—also provides this additional protection and improved availability.

Hot-swap drives and hot-spare drives can further improve availability in critical environments; this is especially true for hot-spare drives. By providing for automatic failover and rebuilding, they can reduce your exposure to catastrophic failure and provide for maximum availability.


The relative performance of each RAID level depends on the intended use. The best compromise for many situations is arguably RAID-5, but you should be suspicious of that compromise if your application is fairly write intensive. Especially for relational database data and index files where the database is moderately or highly write intensive, the performance hit of using RAID-5 can be substantial. A better alternative is to use RAID-0+1 or RAID10.

Whatever level of RAID you choose for your particular application, it will benefit from using more small disks rather than a few large disks. The more drives contributing to the stripe of the array, the greater the benefit of parallel reading and writing you’ll be able to realize—and your array’s overall speed will improve.


The delta in cost between RAID configurations is primarily the cost of drives, potentially including the cost of additional array enclosures because more drives are required for a particular level of RAID. RAID-1, either duplexing or mirroring, is the most expensive of the conventional RAID levels, because it requires at least 33 percent more raw disk space for a given amount of net storage space than other RAID levels.

Another consideration is that RAID levels that include mirroring or duplexing must use drives in pairs. Therefore, it’s more difficult (and more expensive) to add on to an array if you need additional space on the array. A net 36-GB RAID-0+1 array, comprising four 18-GB drives, requires four more 18-GB drives to double in size, a somewhat daunting prospect if your array cabinet has bays for only six drives, for example. A net 36-GB RAID-5 array of three 18-GB drives, however, can be doubled in size simply by adding two more 18-GB drives, for a total of five drives.

Hot-Swap and Hot-Spare Disk Systems

Hardware RAID systems can provide for both hot-swap and hot-spare capabilities. A hot-swap disk system allows failed hard disks to be removed and a replacement disk inserted into the array without powering down the system or rebooting the server. When the new disk is inserted, it is automatically recognized and either will be automatically configured into the array or can be manually configured into it. Additionally, many hot-swap RAID systems allow you to add hard disks into empty slots dynamically, automatically or manually increasing the size of the RAID volume on the fly without a reboot.

A hot-spare RAID configuration uses an additional, preconfigured disk or disks to automatically replace a failed disk. These systems usually don’t support hot-swapped hard disks so that the failed disk can’t be removed until the system can be powered down, but full fault tolerance is maintained by having the hot spare available.

Distributed File System

The Distributed File System (DFS) is primarily a method of simplifying the view that users have of the available storage on a network—but it is also, when configured appropriately, a highly fault-tolerant storage mechanism. By configuring your DFS root on a Windows Server 2003 domain controller, you can create a fault-tolerant, replicated, distributed file system that gives you great flexibility while presenting your user community with a cohesive and easy-to-navigate network file system.

When you create a fault-tolerant DFS root on a domain controller and replicate it and the links below it across multiple servers, you create a highly fault-tolerant file system that has the added benefit of distributing the load evenly across the replicated shares, giving you a substantial scalability improvement as well.


Windows Server 2003 supports two different kinds of high availability clustering, either of which can greatly improve your fault tolerance:

  • For many TCP/IP-based applications, the Network Load Balancing service provides a simple, “shared nothing,” fault-tolerant application server.

  • Server clusters provide a highly available fault-tolerant environment that can run applications, provide network services, and distribute loads. Server clusters are available only with Windows Server 2003, Enterprise Edition, and Windows Server 2003, Datacenter Edition.

Network Load Balancing

The Network Load Balancing service allows TCP/IP-based applications to be spread dynamically across up to 32 servers. If a particular server fails, the load and connections to that server are dynamically balanced to the remaining servers, providing a highly fault-tolerant environment without the need for specialized, shared hardware. Individual servers within the cluster can have different hardware and capabilities, and the overall job of load balancing and failover happens automatically, with each server in the cluster running its own copy of Wlbs.exe, the Network Load Balancing control program.

Server Clusters

Server clusters generally use a shared resource between nodes of the cluster. This resource is generally a shared SCSI or Fibre Channel–attached disk array. Each server in the cluster is connected to the shared resource, and the common database that manages the clustering is stored on this shared disk resource. Nodes in the cluster generally have identical hardware and identical capabilities, although it is technically possible to create a server cluster with dissimilar nodes. Windows Server 2003 supports up to eight node server clusters.

Server clusters provide a highly fault-tolerant and configurable environment for mission-critical services and applications. Applications don’t need to be specially written to be able to take advantage of the fault tolerance of a server cluster, although if the application is written to be clustering aware, it can take advantage of additional controls and features in a failover and fallback scenario.

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