Implementing Software RAID

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1 Implementing Software RAID on Dell PowerEdge Servers Software RAID is an inexpensive storage method offering fault tolerance and enhanced disk read-write performance. This article defines and compares three levels of software RAID, contrasts hardware and software RAID, and details the steps necessary to implement software RAID on Dell PowerEdge servers running the Microsoft Windows Server 23 or Red Hat Linux operating system. BY ROBERT NADON AND THOMAS LUO RAID provides enhanced speed and fail-safe data storage on hard disk drives, making all the hard disk drives on a system appear as a single drive. The Microsoft Windows and Linux operating systems support three levels of software RAID: disk striping (RAID-), disk mirroring (), and disk striping with parity (). Disk striping reads and writes to each disk simultaneously during I/O operations, which improves the performance of diskintensive applications. Disk mirroring provides fault tolerance, but slows performance because the data must be written twice. Disk striping with parity provides both low-cost fault tolerance and improved performance. 1 Figure 1 compares the different software RAID levels. Comparing software and hardware RAID RAID exists in two forms: hardware RAID and software RAID. On Dell PowerEdge servers, hardware RAID is implemented using either a PowerEdge Expandable RAID Controller (PERC) Peripheral Component Interconnect (PCI) card or RAID on Motherboard (ROMB). In hardware RAID, the intelligence of the RAID system is handled by the Disk striping Disk mirroring Disk striping with Software RAID features (RAID-) () parity () Number of disks required Two or more Two Three or more Fault tolerance No Yes Yes Increased performance Yes No Yes Ability to run despite a hard disk failure No Yes No Additional disk space required None Double One parity volume Figure 1. Comparing software RAID features controller and not the operating system (OS). To the OS, the physical disk array appears to be a single hard drive. If one hard disk fails in a or implementation, administrators can rebuild the RAID system and recover all data from the failed disk by accessing the RAID configuration (usually by pressing Control-M during boot) and then selecting Rebuild. These steps recover the data and restore the system to its state immediately before the failure occurred. Software RAID is implemented by the OS, within the kernel disk code, and is independent of the type of hardware in the disk array for example, IDE or SCSI. 1 For more information on RAID, see Understanding and I/O Processors by Paul Luse in Dell Power Solutions, May 23; and Selecting the Best RAID Storage for MSCS Clusters by Mike Kosacek and Victor Mashayekhi in Dell Power Solutions, February 22 Special Issue. POWER SOLUTIONS 77

2 Whereas hardware RAID occurs on an external hard disk array, software RAID resides on the host server volumes; two or more volumes are built into a RAID group. Applications writing to disk must go through the OS, which performs the necessary actions to implement the RAID system. Because software RAID is implemented at the partition level, administrators should be aware of actions that could harm the system. For instance, the main benefit of disk mirroring is fault tolerance; everything written to one partition also is written to a second partition. However, if administrators create a mirror using two partitions that reside on the same hard disk drive, and that drive becomes completely unusable, then all data residing in the mirrored partition will be lost. Advantages of software RAID include: Cost: Because the OS contains all the intelligence, software RAID implementations cost only the time needed to configure the RAID system and the extra disk space required for fault tolerance, if desired. Control: Administrators can control a software RAID setup down to the volume level. This ability improves control over specifying which data is stored in the RAID system and at what RAID level. Implementation: Assuming the necessary free disk space is available, administrators can convert regular volumes to RAID storage while the OS is running and all systems are functioning. However, placing part of the OS itself into a RAID system requires restarting the OS. Disadvantages of software RAID include: Time and CPU cycles: Software RAID is more time-consuming to configure and execute, because the OS creates the RAID system. For example, to perform mirroring, the OS writes to the hard disk drives twice. Software RAID performs each write sequentially, whereas hardware RAID performs the writes simultaneously. Reliability: Because an entire disk fails more often than a portion of a disk fails, performing fault tolerance is more reliable at the disk level (hardware RAID) than at the partition level (software RAID). Vulnerability: If the OS becomes corrupted and unusable, the OS may corrupt the software RAID configuration and all data could be lost. In contrast, a corrupted OS does not corrupt hardware-based RAID. server, such as a file allocation table (FAT) or the NT file system (NTFS). Volumes comprise storage areas on a hard disk. volumes. RAID- striped volumes divide data into blocks, spread equally over two or more hard disks. Striping reads or writes the files across all disks so that data can be read or written to all disks at the same rate. Windows Server 23 requires at least two, and can use as many as 32, disks (or free areas on the disks) to implement a striped volume. A striped volume does not provide fault tolerance; if one disk fails, the whole volume fails with no data recovery. volumes. mirrored volumes create a shadow copy of data on a selected volume. Data that is written to the primary volume also is written to a secondary volume (see Figure 2). If one disk fails, the system uses data from the other disk. The performance drawback is that mirroring doubles the time to create, update, and store data. volumes with parity. volumes share data among disks in an array. This approach generates parity information in each disk of an array, and uses parity to reconstruct lost information should disks fail. requires at least three disks. As more disks are added, the overhead decreases from a maximum of 5 percent (that is, three disks are required to store the data normally contained on two disks) to a minimum of 3 percent using 32 disks. Comparing basic and dynamic disks Administrators must understand the distinction between basic and dynamic disks when implementing software RAID on Microsoft operating systems. A basic disk is similar to the logical disk configuration used in the Microsoft Windows NT OS, which uses partitions to organize the disk into storage units. Operating systems such as Windows 2, Windows NT, Windows 9x, and DOS can access basic disks. A dynamic disk is a physical disk that does not use partitions or logical drives; instead, it uses volumes to organize the disk. Regardless of the file system format, only Windows 2 or Windows Server 23 based servers can access dynamic volumes directly. However, other systems can access the dynamic volumes remotely when connected to either shared folders or the network. Administrators can use dynamic disks to create fault-tolerant volumes (such as mirrored volumes) and volumes. In addition, administrators can extend volumes and make changes to the disk without rebooting the system. Disk 1 (primary volume) Implementing software RAID on Windows Server 23 Administrators implement software RAID on Microsoft Windows Server 23 by formatting volumes using the file system of the host Figure 2. volumes in a configuration Disk 2 (secondary volume) 78 POWER SOLUTIONS August 23

3 NTFS, FAT32, and FAT16 file systems all are available on a basic or a dynamic disk. Because the upgrade from basic to dynamic occurs at the physical disk level, all volumes on a physical disk must be either basic or dynamic. Dynamic disks can accommodate volumes that span multiple disks (spanned and striped volumes), and can provide fault tolerance (mirrored and volumes). All volumes on dynamic disks also are referred to as dynamic volumes. When Windows Server 23 is installed, the system initially configures the hard disks as basic disks. Basic disks cannot be extended. However, administrators can convert the basic disks to dynamic disks by using either the Microsoft Disk Management snap-in or the new Microsoft Diskpart command-line utility. Once converted, the dynamic volumes can be extended. A dynamic disk has its own limitations. Windows Server 23 cannot be installed on a dynamic volume that was created from raw space on a dynamic disk. However, administrators can install Windows Server 23 on a dynamic volume that was upgraded from a basic disk, but they cannot extend the system or the boot partition. Administrators should exercise caution when converting dynamic disks to basic disks. Because the dynamic volumes cannot be changed back to partitions, administrators should move or back up data and delete all dynamic volumes on the disk before converting the disk. Selecting the best software RAID strategy The best software RAID strategy depends on the IT budget and computing environment. RAID- striped volumes provide highspeed reads and writes, which is advantageous for I/O-intense applications such as imaging and scientific analysis. However, striped volumes do not provide fault tolerance. mirrored volumes do not suffer performance degradation if the OS fails. volumes incur lower startup costs, require less system memory, and provide better overall performance. mirrored volumes may work best in environments where data is frequently updated. However, if the storage overhead cost of a mirrored volume is prohibitive, administrators can achieve redundancy using volumes, which also provide fault tolerance, and at a lower cost per megabyte. volumes offer disk striping with parity, which achieves excellent read performance as well as fault tolerance at a lower cost per megabyte than mirrored volumes. Administrators also can improve system performance by locating swap files on the volume. However, the performance of a volume suffers when a disk in the array fails. In addition, volumes require more system memory than RAID- and volumes, and OS and boot files cannot reside on volumes. In short, the major differences among striped, mirrored, and striped-with-parity volumes are hardware requirements, performance, and cost. Figure 3 further illustrates these differences. Content and volumes volumes Volumes striped environment (RAID-) () with parity () FAT16, FAT32, and NTFS file systems Yes Yes Yes System and boot volume striping support No Yes No Minimum number of hard disks required Two Two Three Cost per megabyte Low High (5 percent utilization) Low Read performance Excellent Good Excellent Write performance Excellent Good Moderate System memory requirements Significant Minimal Significant Fault tolerance No Yes Yes Number of hard disks supported Up to 32 Up to 2 Up to 32 Figure 3. Comparing content and environment among striped, mirrored, and striped-with-parity volumes Creating RAID-,, or volumes in Windows Server 23 To create striped, mirrored, or striped-with-parity volumes in Windows Server 23, administrator privileges and dynamic disks are required (see the Upgrading basic disks to dynamic disks section of this article for more information). Administrators can follow these steps: 1. Click Start, point to Administrative Tools, then click Computer Management. 2. Expand the plus sign next to Storage to open the Storage console tree. 3. Click Disk Management. 4. Right-click the unallocated space on one of the dynamic disks where the RAID-,, or volume should be created, and then click Create Volume. 5. In the Create Volume wizard, click Next, and then click on the desired volume type: volume, volume, or volume. 6. Follow the instructions on the screen. Upgrading basic disks to dynamic disks To upgrade basic disks to dynamic disks, administrators can perform the following steps: 1. Close all programs and applications that are running on the disks to be upgraded. 2. Right-click the gray Disk Description pane that is located to the left of the color-coded volume panes. 3. Click Upgrade to Dynamic Disk. 4. If the next disk is not a dynamic disk, repeat steps 2 and 3 to upgrade it. 8 POWER SOLUTIONS August 23

4 Automating software RAID by using Diskpart Microsoft provides a disk-partitioning utility called Diskpart. Administrators can use Diskpart to create scripts that automate such tasks as creating mirrored or volumes and converting basic disks to dynamic disks. Scripting these tasks is useful when deploying unattended setup for Windows operating systems. Implementing software RAID on Red Hat Linux As in Microsoft operating systems, administrators can implement software RAID in Red Hat Linux using partitions instead of actual hard disks. A Linux partition is similar to a Windows volume in that it is a section of the hard drive. Linux partitions reside in a directory, similar to the C: drive in Windows. These partitions can be logical volumes spanning multiple drives or static partitions confined to a single drive. Either logical volumes or static partitions can be included within the software RAID configuration for Linux. In Linux, administrators must build the RAID configuration when they create the partitions. Administrators usually perform this task during installation; however, if the RAID implementation is desired for partitions unnecessary to the OS, it can be built at any time. Red Hat Linux divides data into chunks (usually 16 KB or 32 KB), and each chunk can then be striped across the partitions. To implement software RAID on Linux, administrators can use one of three methods: Fdisk, an MS-DOS disk partitioning tool; Disk Druid, a graphical user interface (GUI) based partitioning tool; or ks.cfg, an automated answer file. Fdisk. To create a software RAID based partition, administrators can use the n directive. After creating the partition, administrators should toggle the type so that it is not hexadecimal by entering fd, which represents the Linux RAID Auto type. Next, administrators should format the partition using the mkraid command. This command needs a configuration file similar to the following: raiddev /dev/md raid-level nr-raid-disks 2 persistent-superblock chunk-size 8 device /dev/sda1 raid-disk device /dev/sdb1 raid-disk 1 GUI. The Disk Druid GUI offers several partition types, including swap, ext3, and software RAID. By creating two software RAID partitions and then clicking the RAID button, administrators can combine these partitions into a RAID- or configuration and assign this RAID level to any partition (such as /). If administrators create three partitions, the three partitions can be combined into a RAID- or configuration. Automated answer file. Administrators also can use the answer file (ks.cfg) to automate the creation of RAID containers during OS installation. For example, to create two partitions in a configuration, an administrator could use the following definition: part raid.1 size 1 ondisk sda part raid.2 size 1 ondisk sdb part /boot level=raid1 raid.1 raid.2 To create three partitions in a configuration, administrators could use the following definition: part raid.11 size 1 ondisk sda part raid.12 size 1 ondisk sdb part raid.13 size 1 ondisk sdb part / level=raid5 raid.11 raid.12 raid.13 Testing RAID performance The Dell team tested read, write, and read-and-write performance of RAID-,, and on Windows 2 and Windows Server 23 operating systems. The team configured the hardware identically for all tests: a Dell PowerEdge 46 server with dual Intel Xeon processors at 2. GHz, 3.5 GB error-correcting code (ECC) double data rate (DDR) SDRAM, and ten 1, rpm hard disk drives. The team timed read, write, and read-and-write operations for each type of RAID and both operating systems. Figures 4, 5, and 6 report the test results, which mimic usage patterns representing common applications in a range of file sizes: 6 KB, 6 KB, and 6 KB. For example, a Web server largely depends on read performance; a file server depends on write performance; and a database application depends on both read and write performance. If all-around performance is a major concern, upgrading to Windows Server 23 is recommended Windows Server 23 outperformed Windows 2 in almost every test. Still, IT professionals can choose the most effective operating system and RAID configuration for their specific price/performance requirements. Although Windows Server 23 outperformed Windows 2 significantly in both read and read-and-write tests, write performance was fairly comparable between the two operating systems. POWER SOLUTIONS 81

5 Achieving cost-effective data protection using software RAID Software RAID provides a fault-tolerant disk management system at no extra cost, allowing all hard drives to appear as a single drive and continuing to perform through hard disk failures. In addition, Data blocks read per second RAID- Windows Server 23 Windows 2 with parity Figure 4. Average data blocks read per second on Windows Server 23 and Windows 2 under certain circumstances software RAID achieves a performance increase over non-raid disk arrays, because the OS can write to and read from multiple disks at the same time. As IT organizations endeavor to do more with less, they can benefit from using software RAID to increase system performance, provide fault tolerance, or both. Although slower than hardware RAID, software RAID offers an inexpensive alternative that enables IT organizations to protect their data cost-effectively. References Hogge, Steve and Bo Zhou. Understanding Server-Based RAID Using Ultra ATA Technology. Dell Power Solutions, March power_ps1q2-hogge.htm Kosacek, Mike and Victor Mashayekhi. Selecting the Best RAID Storage for MSCS Clusters. Dell Power Solutions, February 22, Special Edition. power_ps4q1se-raid.htm 35 Windows Server 23 Windows 2 Luse, Paul. Understanding and I/O Processors. Dell Power Solutions, May Data blocks written per second topics/power_ps2q3-luse.htm Molloy, Mike and Serdar Acir. PERC 3/QC Array Storage Controller. Dell Power Solutions, Issue 3, Molloy, Mike and Serdar Acir. Performance Characteristics of PERC 3/QC Release 1.2 RAID Controller. Dell Power RAID- with parity Solutions, March topics/power_ps1q2-perc.htm Figure 5. Average data blocks written per second on Windows Server 23 and Windows 2 Data blocks read and written per second RAID- Windows Server 23 Windows 2 with parity Robert Nadon (robert_nadon@dell.com) is a software engineer consultant in the Factory Installation Development Group at Dell. He focuses on factory installation for UNIX -based (Linux and BSDi) operating systems and for the Oracle9i database. Another area of interest is Dell software RAID offerings. Robert has a B.S. in Computer Engineering from Texas A&M University. He is a Red Hat Certified Engineer (RHCE ) for Red Hat Linux 6.2 and 7.. Thomas Luo (thomas_luo@dell.com) is a software engineer and consultant in the Factory Installation Development Group at Dell. He has a bachelor s degree in Applied Physics from Hua Qiao University, China, and an M.B.A. from St. Edward s University, Austin, Texas. Thomas is a Microsoft Certified Systems Engineer (MCSE) and a Certified Novell Engineer (CNE). FOR MORE INFORMATION Dell storage: Figure 6. Average data blocks read and written per second on Windows Server 23 and Windows 2 82 POWER SOLUTIONS August 23

The term "physical drive" refers to a single hard disk module. Figure 1. Physical Drive

The term physical drive refers to a single hard disk module. Figure 1. Physical Drive HP NetRAID Tutorial RAID Overview HP NetRAID Series adapters let you link multiple hard disk drives together and write data across them as if they were one large drive. With the HP NetRAID Series adapter,