Dell PowerVault 110T LTO2 User Manual [en, de, es, fr]

Dell™ PowerVault™ Systems

Performance Considerations

for Tape Drives and Libraries

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____________________

Reproduction in any manner whatsoever without the written permission of Dell Inc. is strictly forbidden. Trademarks used in this text: Dell, the DELL logo, and PowerVault are trademarks of Dell Inc.; EMC and PowerPath are registered trademarks

of EMC Corporation. Other trademarks and trade names may be used in this document to refer to either the entities claiming the marks and names or their products.

Dell Inc. disclaims any proprietary interest in trademarks and trade names other than its own.

June 2005

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

General Host Backup Considerations

Tape Drive and Data Considerations Hard Drive and RAID Array Configuration

General Performance Considerations When Using Multiple Drives in Tape Libraries

SCSI Configurations SAN Configurations

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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Contents 3

4 Contents

Introduction

ith recent improvements in tape drive transfer rates, many host-side factors, such as RAID (Redundant Array of Inexpensive [or Independent] Disks) configuration and hard-drive specifications, must be considered when determining whether the host server and tape drive can process data at the same rate. General configurations and attributes that may limit throughput from the host server to the tape drive are discussed in "General Host Backup Considerations."

As multiple drives are placed into tape libraries, greater host bandwidths are needed to keep pace with the potential throughput of multiple tape drives. Potential fibre limitations for multidrive units, as well as recommended cabling configurations, are discussed in "General Performance Considerations When Using Multiple Drives in Tape Libraries."

General Host Backup Considerations

The considerations in this section apply to both SCSI and storage area network (SAN) tape backup configurations.

Tape Drive and Data Considerations

The following issues should be considered when evaluating performance:

Overhead from SCSI commands.

in achieving theoretical maximum transfer speeds. Tape backup software does not account for this overhead; instead, the software only measures the rate at which data is written to the tape. For example, the drive may be processing 80 MB/sec of data, but only writing 77 MB/sec of data. The latter rate is what the backup software will report.

Tape block sizes.

applications allow the user to change block size, even though a larger size will not enhance performance. Using block sizes less than 64 Kb can actually hinder performance. See your backup software User's Guide for information on adjusting the block size of your tape device.

Backup software buffer size.

as large as possible. Some applications allow users to change the buffer size, which can help maintain a steady stream of data to and from the drive and significantly increase transfer rates, especially of small files. The larger the buffer, the more data it can hold and the less time the disk spends seeking the data; however, this can affect memory and CPU performance. See your tape backup application User’s Guide for specific details.

Drivers and firmware.

drivers and firmware installed. Visit for your Dell PowerVault tape product.

64 Kb block sizes are optimal for most tape drives. However, some backup

Always ensure that the SCSI or fibre controller and tape drive have the latest

Command overhead on the SCSI bus restrict

For optimal backup performance, backup software buffers should be

support.dell.com

to download the latest drivers and firmware

all

SCSI devices

Performance Considerations for Tape Drives and Libraries 5

Attach tape drives and hard drives on separate controllers (internal or external) on separate host bus adapters (HBAs).

but best practice is to keep the tape drive HBA separate from the hard drive HBA to ensure maximum throughput. Most onboard dual-mode SCSI/RAID controllers share one processor, which must share bandwidth between the RAID array and tape drive. Thus, one controller is handling reads and writes between the hard disks and the tape drive, as well as calculating and writing any necessary parity information to the hard drives. See "Hardware RAID Configuration Considerations" for specific information on RAID arrays and parity bytes.

Dirty drive heads or old media.

a corresponding reduction in read/write speeds. Each time a drive attempts to rewrite or reread a block on a tape, performance is degraded. Once a certain threshold of read/write errors is reached,

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the drive will usually request cleaning. It is important to clean the drive heads on regular intervals, or when requested.

The chance of encountering a bad block increases as media ages or is excessively used. The typical lifecycle of a piece of LTO media is approximately 75 full tape read/writes.

Speed matching.

down to approximately one-half of the maximum uncompressed data transfer speed. If data is provided to the drive at less than the lower speed matching limit, the drive must stop, wait for the buffer to fill, rewind, and then attempt to write the buffer (this is known as "back hitching").

For example, the Dell PowerVault 110T (LTO2 and LTO3) tape drive matches speed down to 30 MB/sec while writing to LTO-3 media. If the host server can only provide data at 20 MB/sec, the drive will "back hitch" while waiting for its buffers to fill. In this situation, the effective throughput will be something less than 20 MB/sec (probably closer to 15 MB/sec).

This depends somewhat on the performance capabilities of your controller,

A dirty tape drive head or old media can cause high error rates and

Newer LTO drives will match the speed of the data being provided to the drive,

Confirming Performance of Your Tape Drive

Certain tape drive manufacturers have a performance diagnostic mode built into the drive that can be used to confirm throughput. The PowerVault 110T LTO-2 and LTO-3 (firmware 53 offer a diagnostic mode "F," which performs a quick read/write performance test on the drive and media. If the performance rate is not within 6 percent of the maximum specified drive speed, the test fails with an error message. No error message is displayed if the test passes. Consult your tape drive User’s Manual for specific details on diagnostic mode "F."

NOTICE: Diagnostic mode "F" requires media that can be safely overwritten as part of the diagnostic test.

Do not use media containing critical data. Any data residing on the media used in the diagnostic test will be lost.

Hard Drive and RAID Array Configuration

Several hard drive and disk array (both internal and external) attributes can affect backup or restore performance. These attributes, as well as recommended configurations that help achieve maximum backup and restore speeds, are discussed in the following subsections. If the tape drive’s sustainable throughput exceeds that of the disk array, then the tape drive’s peak performance will not be realized.

6 Performance Considerations for Tape Drives and Libraries

or later)

General Hard Drive Configuration Considerations

Data/operating system (OS) on different LUNs.

Backing up data on a logical unit number (LUN) separate from the OS LUN ensures that the hard drive is not splitting access and overhead between OS operations and backup operations. This can be accomplished by having one hard drive or disk array contain the OS and a physically separate hard drive or disk array contain the data to be backed up.

Figure 1-1. Single-Channel vs. Two-Channel Bandwidth

Single Shared LUN Separate LUNs

Single LUN with

Backup data

and OS

OS LUN

Backup Data

LUN

SCSI or RAID

Controller

Tape Drive

Hard Drive Performance

SCSI or RAID

Controller

Tape Drive

By design, tape drives write data sequentially and require a constant data feed to keep the drive operating sequentially (avoiding back hitching). Conversely, hard drives are random access devices. Therefore, hard drives can sometimes struggle to provide sequential data to tape drives if that data is spread out over the drive platter. This forces the drive to continuously seek small blocks of data.

Additionally, other hard drive attributes can further affect the throughput of data to the tape drive.

Spindle speed.

Typically measured in RPMs (revolutions per minute), the hard drive's spindle speed determines how many times per minute the drive platter assembly can perform a full revolution. This has a direct effect on both random access times and sequential transfer rates. The higher the spindle speed, the faster the drive can access data.

Performance Considerations for Tape Drives and Libraries 7

Random access time or seek time.

time a drive's heads take to find a piece of data on the disk. The seek time of a hard disk measures the amount of time required for the read/write heads to move between tracks on the surface of the platters. Because hard disks are random access devices, data can be stored on virtually any sector of the disk. The longer it takes to access that data, the slower the overall throughput of the drive. This attribute is very significant when a hard drive contains many small files. The smaller the files, the more "seeks" the drive must make to read or write the file to disk; therefore, disks tend to read or write very slowly when many small files are being transferred.

Sequential/sustained transfer rates (STR).

and writes data to its platters. If the data being backed up is one large contiguous file, the sustained throughput will be close to the drive's maximum STR. However, in real-world applications, data

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becomes more scattered about the platter as data is deleted and written. Defragmenting a hard drive can help the drive reach its maximum STR.

Buffer (cache).

written or stored data. The bigger the buffer, the more data it can hold, resulting in less time seeking data on the disk.

Hardware RAID Configuration Considerations

General overview of RAID

This section presents an overview of typical RAID configurations and how they affect backup and restore rates. A RAID array is a set of hard disks that act as a single storage system or LUN. Data can be potentially transferred through the channel of each hard drive at once, allowing for total throughput to be a multiple of the total number of drives in the array, minus overhead and any redundancy as described in the following sections.

In the case of a RAID configuration, the speed of the interface becomes important because the drives share the bandwidth of the interface. For example, a single Ultra160 drive may only sustain 40 MB/sec. Thus, a five-disk RAID 0 array consisting of the same drive type should be able to read/write at 200 MB/sec. However, the Ultra160 interface will limit the array to a maximum of 160 MB/sec.

External disk arrays, particularly in SANs, may offer significant levels of cache memory to improve I/O performance. This cache will greatly improve performance when writing to the array and may store frequently accessed data to improve read performance. With respect to its impact on tape performance, the cache will mask most RAID limitations when restoring data to the array or backing up data from the array. However, backup operations from external arrays with cache may still feel the impact of RAID configuration limitations because the data still needs to be read from the disks.

Usually measured in milliseconds, seek time is the length of

STR measures how fast a drive actually reads data from

The buffer is the amount of memory on the drive that holds the most recently

8 Performance Considerations for Tape Drives and Libraries

RAID 0

Commonly known as striping, RAID 0 allows two or more disks to be joined to create one virtual drive in the fashion of a single LUN. It is referred to as striping because data is written across all of the disks in the array, not just to one disk at a time. Thus, the throughput is spread across channels (

being the number of hard drives in the array) instead of a single channel for a single

hard disk. This results in excellent read/write performance, but no fault tolerance.

Figure 1-2 shows four hard drives in a RAID 0 configuration. Data is striped across all four hard drives, resulting in four channels for reading and writing to the array.

Figure 1-2. Example RAID 0 Configuration

Hard Drive 1 Hard Drive 2 Hard Drive 3 Hard Drive 4

D = Data Byte

D13

D17

D10

D14

D18

SCSI or RAID

Controller

Tape Drive

D11

D15

D19

D12

D16

D20

Performance Considerations for Tape Drives and Libraries 9

RAID 1

Also known as mirroring, RAID 1 means data is written twice across two disks simultaneously. If one drive fails, the system switches to the other drive without losing data. During tape drive backups, the read rate from the RAID 1 array is approximately the same as a single drive because it is reading from the primary drive. However, restore performance from the tape drive writing to the RAID 1 array can be slower due to error checking/correction (ECC) included in writing to the primary and mirrored disks. Much of this inefficiency is due to the fact that the mirroring is often performed on the CPU or RAID controller. Thus, newer RAID controllers tend to be faster due to newer and more capable processors.

Figure 1-3. Example RAID 1 Configuration

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D = Data Byte

M = Mirrored Byte

Hard Drive 1 Hard Drive 2

SCSI or RAID

Controller

Read/Write

Tape Drive

Write OnlyRead/Write

10 Performance Considerations for Tape Drives and Libraries

RAID 5

With a RAID 5 array, data is striped across the disk array at the byte level and error correction data, or parity data, is also striped across the disk array. RAID 5 arrays tend to have very good random read performance; this read performance generally improves as the number of disks in the array increases. With the larger disk arrays, read performance can actually outperform RAID 0 arrays because the data is distributed over an additional drive. In additional, parity information is not required during normal reads.

Restores from tape to a RAID 5 array tend to be nominal because it involves additional overhead for calculating and writing the parity information.

Figure 1-4. Example RAID 5 Configuration

Hard Drive 1 Hard Drive 2 Hard Drive 3 Hard Drive 4 P = Parity Byte

D = Data Byte

Read/Write

D10

D13

D11

D14

SCSI or RAID

Controller

Read/Write

Tape Drive

D12

D15

D16

Performance Considerations for Tape Drives and Libraries 11

In conclusion, RAID 0 tends to be the best overall configuration for read and write performance, but does not allow for redundancy. RAID 1 is the worst performer overall, as all data written to the array is mirrored and reads come from a single disk. RAID 5 tends to be a good read performer but average write performer; however, RAID 5 improves if more disks are added to the array. If the RAID is within an enclosure that offers significant levels of cache memory, then performance limitations during restore operations may be abated. Backup operations will still be subject to limitations of the RAID configuration. heavily on the specific hard drive characteristics listed in "Hard Drive Performance."

In addition, the characteristics of the array still depend

General Performance Considerations When Using Multiple

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When multiple tape drives are utilized simultaneously to perform data backups (such as in tape libraries), additional aspects of the hardware configuration must be considered. By employing simple performance-minded methods in setting up hardware and cabling configurations, additional throughput bottlenecks can be limited.

SCSI Configurations

The latest high-performance tape drives offered in tape libraries support the Ultra160 specification of the SCSI interface standard. Therefore, to achieve maximum performance, backup servers utilizing SCSI must have an HBA installed that supports data speeds of Ultra160 or higher. A SCSI HBA that meets this requirement will allow each tape drive to communicate with the host at a rate of 160 MB/sec on the SCSI bus. The higher data rate of the SCSI bus compared to tape drive speeds allows multiple devices to be connected to the same bus without sacrificing device performance. But only to a point.

The 160 MB/sec data rate of an Ultra160 bus is the maximum possible data throughput rate to all devices connected to the bus. Therefore, a single tape drive will not consume the full bandwidth of the bus because it can read or write data to tape at up to 80 MB/sec (native). Multiple tape drives, however, can combine to consume the full 160 MB/sec offered by the bus if each is operating at its maximum native performance. Each additional drive connected to the same bus after this point will reduce the average performance of each drive.

Therefore, to achieve maximum performance from a tape library, it is recommended to connect no more than two tape drives to each SCSI bus. See "Recommended Cabling Configurations" for specific details and illustrations. A SCSI HBA supporting at least Ultra160 should be used, but upgrading to an Ultra320 HBA will not lead to an additional improvement in performance if the tape drive's specification is Ultra160.

Drives in Tape Libraries

12 Performance Considerations for Tape Drives and Libraries

SAN Configurations

Fibre Channel (FC) offers many advantages over SCSI. First, it overcomes the distance limitations of SCSI (12 meters for LVD SCSI versus 300 meters for a short-wave 2-Gb FC link) the transmission of data at higher speeds. As a serial network protocol rather than a bus-based architecture like SCSI, FC has also become the protocol of choice for the implementation of SANs, allowing for the consolidation of data storage resources. In addition, each FC connection is made up of a transmit link and a receive link, allowing for full-duplex operation. This means that data can be transmitted in two directions simultaneously. Therefore, during a backup operation across a single FC connection, data can be read from a source and written to tape without taking turns in communication, effectively doubling the bandwidth of a connection. See Figure 1-5.

Figure 1-5. Fibre Channel Link Diagram

2 Gb = 200 MB/sec

and allows for

HOST

Transmit

Receive

Transmit

2 Gb = 200 MB/sec

Fibre

Channel

Device

When setting up tape libraries in a SAN, performance can still be affected by various factors. These factors include FC link speeds, data flow between the source and tape library, and performance limitations of external storage arrays. With an understanding of the overall setup and management of the solution, many of these factors can be avoided.

Even with the high data bandwidth offered by the FC protocol in SANs, proper considerations must be made for tape drives in order to avoid a situation in which the FC link may limit performance. The data rate of a 2-gigabit (Gb) FC link is 200 MB/sec (that is, 200 MB/sec on the transmit link and 200 MB/sec on the receive link). Therefore, attempting to operate multiple tape drives across the same link can potentially exceed the full bandwidth of a link. If the host is operating with a legacy 1-Gb adapter, backing up data to two drives may be sufficient to reveal significant performance limitations.

Therefore, when using three or more tape drives simultaneously on a 2-Gb link, you may need to distribute the backups across a number of connections, rather than relying on a single link. This is where understanding the SAN solution's topology is beneficial. Following the data path during a backup operation as it is read from the source and then written out to tape will help administrators recognize any potential bottlenecks. If any bottlenecks are identified, measures may be taken depending on the configuration. For example, if the backup solution requires multiple drives to be in operation at once, splitting the tape hardware across separate fabrics may improve performance by splitting the connections. See Figure 1-6.

Performance Considerations for Tape Drives and Libraries 13

Figure 1-6. Single vs. Split Data Flow to Tape Library

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Tape Library Tape Library

At the same time, if a bottleneck exists in the data being read from an external FC disk array, the use of I/O management software such as EMC will automatically load-balance the data across multiple paths and increase availability through path failover. See Figure 1-7. The left side of the figure represents a SAN disk array in which all of the data is forced through a single link, creating a bottleneck that slows data flow to the tape hardware. The right side shows how load balancing doubles the I/O bandwidth coming out of the array by allowing the data to transmit across two links.

HOST

Fibre Channel

Switch

Fibre Channel

Disk Array

HOST

Fibre Channel

Switch

PowerPath® with an additional fabric connection

Fibre Channel

Switch

Fibre Channel

Disk Array

14 Performance Considerations for Tape Drives and Libraries

Figure 1-7. Bottlenecked Data Flow vs. Load-Balanced Disk Array

HOST

Fibre Channel

Switch

Tape Library Tape Library

Fibre Channel

Switch

Fibre Channel

Disk Array

Fibre Channel

Switch

HOST

Fibre Channel

Switch

Fibre Channel

Disk Array

Finally, FC disk arrays on the SAN can also experience the same performance limiters described in "Hard Drive and RAID Array Configuration." Therefore, improving the performance characteristics of the disk arrays will also have a direct effect on backup speed across the SAN.

SAN Configurations Utilizing the Library Fibre Channel Bridge

Certain tape libraries may be connected to a SAN by way of a Fibre Channel bridge module. The module acts as a bridge between the SCSI and FC protocols and provides additional management, security, and operational features unavailable in most native FC libraries. For details on these features, see the Fibre Channel bridge User's Guide for your tape library.

In some tape library configurations, the Fibre Channel bridge module may act as a bottleneck and decrease performance of tape drives. This is because the processing capability in the Fibre Channel bridge module required to bridge the SCSI and FC communication cannot meet the aggregate data throughput offered by certain multidrive configurations. Despite this, most data backup solutions will not experience the Fibre Channel bridge module as the primary limiting factor in tape performance. Dedicated backup servers will frequently encounter a situation where the limitations at the host will be compounded by the exertion of feeding data to multiple tape drives. This results in average drive performance below the level where the Fibre Channel bridge module becomes a factor.

Performance Considerations for Tape Drives and Libraries 15

Recommended Cabling Configurations

SCSI Cabling to the Host

Tape Library With up to Six Tape Drives

When the tape library is SCSI-attached to a host, ensure that no more than two drives are on a single bus. Additional SCSI controllers are required for libraries with five or six tape drives to ensure that no SCSI bus becomes a barrier to maximizing throughput.

Figure 1-8. SCSI Cabling for Library With up to Six Tape Drives

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library-to-host SCSI cable

drive-to-library controller SCSI cable

terminator

Tape Library With up to Two Tape Drives

The drives in a fully configured two-drive tape library can be cabled to a host on the same SCSI bus without encountering significant limitations to backup performance. The backup rates for two drives on a single SCSI bus will match the backup rates for two drives on separate buses. However, customers who enable the verify feature in backup applications may wish to improve the verify performance by splitting two drives onto two SCSI buses. By doing so, verify performance may improve by up to 25 percent.

16 Performance Considerations for Tape Drives and Libraries

Figure 1-9. SCSI Cabling for Library With up to Two Tape Drives

terminator

Cabling Drives to the Fibre Channel Bridge

Figures 1-10 through 1-17 illustrate how a tape library with up to six drives should be configured with a Fibre Channel bridge module in order to optimize tape performance over FC.

Performance Considerations for Tape Drives and Libraries 17

Figure 1-10. Fibre Channel Bridge Cabling With One Tape Drive

library SCSI interface

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SCSI 1

terminator

drive 1

18 Performance Considerations for Tape Drives and Libraries

Figure 1-11. Fibre Channel Bridge Cabling With Two Tape Drives

library SCSI interface

SCSI 1

terminator

drive 2

terminator

drive 1

SCSI 2

Performance Considerations for Tape Drives and Libraries 19

Figure 1-12. Fibre Channel Bridge Cabling With Three Tape Drives

library SCSI interface

terminator

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SCSI 1

SCSI 2

drive 3

drive 2

terminator

drive 1

20 Performance Considerations for Tape Drives and Libraries

Figure 1-13. Fibre Channel Bridge Cabling With Four Tape Drives

library SCSI interface

SCSI 1

SCSI 2

terminator

drive 4

drive 3

terminator

drive 2

drive 1

Figure 1-14. Channel Zoning Settings for Tape Library With One to Four Drives

Performance Considerations for Tape Drives and Libraries 21

Figure 1-15. Fibre Channel Bridge Cabling With Five Tape Drives

terminator

library SCSI interface

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SCSI 3

SCSI 4

SCSI 1

SCSI 2

drive 5

drive 4

terminator

drive 3

drive 2

terminator

drive 1

22 Performance Considerations for Tape Drives and Libraries

Figure 1-16. Fibre Channel Bridge Cabling With Six Tape Drives

terminator

drive 6

library SCSI interface

SCSI 3

SCSI 4

SCSI 1

SCSI 2

Figure 1-17. Channel Zoning Settings for Tape Library With Five or Six Drives

terminator

drive 5

drive 4

terminator

drive 3

drive 2

terminator

drive 1

Performance Considerations for Tape Drives and Libraries 23

Figure 1-18 and Figure 1-19 illustrate how a tape library with up to two tape drives should be configured with a Fibre Channel bridge module in order to optimize tape performance over FC.

Figure 1-18. Tape Library With One Drive

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Figure 1-19. Tape Library With Two Drives

terminator

24 Performance Considerations for Tape Drives and Libraries

Dell™ PowerVault™ 系统

磁带驱动器和磁带存储库

性能注意事项

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____________________

未经 Dell Inc. 书面许可，严禁以任何方式进行复制。

本文中使用的商标：Dell、DELL 徽标和 PowerVault 是 Dell Inc. 的商标； EMC 和 PowerPath 是 EMC Corporation 的注册商标。

本文件中述及的其它商标和产品名称是指拥有相应商标和名称的公司或其制造的产品。Dell Inc. 对本公司的商标和产品名称之外的其它商标和产品名称不拥有任何专有权。

2005 年 6

月

简介 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

主机备份一般注意事项

磁带驱动器和数据注意事项硬盘驱动器和 RAID 阵列配置

在磁带存储库中使用多个驱动器时的一般性能注意事项

SCSI 配置 SAN 配置

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . 29

. . . . . . . . . . . . . . . . . . . . . . 30

. . . . . . . . .

目录 27

28 目录

简介

随着近来磁带驱动器传输速率的提高，在确定主机服务器和磁带驱动器能否以相同速率处理

RAID

数据时，必须考虑许多主机方面的因素，如盘驱动器的规格。在“主机备份一般注意事项”中，将讨论可能会限制从主机服务器到磁带驱动器吞吐量的一般配置和属性。

由于磁带存储库中放入了多个驱动器，因此，为了满足多个磁带驱动器的潜在吞吐量要求，需要增加主机带宽。“在磁带存储库中使用多个驱动器时的一般性能注意事项”中将讨论有关多驱动器单元的潜在光纤限制以及建议的布线配置。

（廉价[或独立]磁盘冗余阵列）配置和硬

主机备份一般注意事项

本节中的注意事项同时适用于

SCSI

和存储区域网络

(SAN)

磁带备份配置。

磁带驱动器和数据注意事项

在评估性能时应考虑以下问题：

SCSI

命令所带来的额外开销。使之无法获得理论上的最大传输速率。磁带备份软件并未说明这一额外开销，而只是测量数据写入磁带的速率。例如，驱动器可能以为

77 MB/

磁带区块大小。对于大多数磁带驱动器，序允许用户更改区块大小，即使较大的区块并不能提高性能。如果区块大小低于则的确会妨碍性能。有关调整磁带设备的区块大小的信息，请参阅备份软件的用户指南。

备份软件缓冲区大小。为达到最佳的备份性能，备份软件缓冲区应尽可能大。某些应用程序允许用户更改缓冲区大小，这样可能有助于在将数据传出和传入驱动器时保持稳定的数据流，从而显著提高传输速率，特别是对于小文件。缓冲区越大，存储的数据就越多，磁盘寻找数据所需的时间就越少；但这样可能会影响内存和备份应用程序的用户指南。

驱动程序和固件。始终确保请访问

在各个主机总线适配器器。这在一定程度上取决于控制器的性能，但最佳的方法是保持磁带驱动器

HBA

器器，这就意味着必须在硬盘与磁带驱动器之间的读和写操作，又要计算和向硬盘驱动器写入任何需要的奇偶校验信息。有关

秒。后一个速率就是备份软件将报告的速度。

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分离，以确保获得最大吞吐量。大多数板载双模式

RAID

阵列和奇偶校验字节的具体信息，请参阅“硬件

，为您的

(HBA)

RAID

SCSI

总线上的命令所带来的额外开销限制了所有

80 MB/

64 KB

SCSI

或光纤控制器及磁带驱动器安装了最新的驱动程序和固件。

Dell PowerVault

上的各个控制器（内部或外部）上连接磁带驱动器和硬盘驱动

阵列与磁带驱动器之间共享带宽。因此，一个控制器既要处理

秒的速度处理数据，但写入数据时的速度仅

的区块大小是最佳的。然而，某些备份应用程

CPU

性能。有关详细信息，请参阅磁带

磁带产品下载最新的驱动程序和固件。

SCSI/RAID

控制器共享一个处理

RAID

配置注意事项”。

HBA

SCSI

64 KB

与硬盘驱动

设备，

，

磁带驱动器和磁带存储库性能注意事项 29

有污垢的驱动器磁头或旧的介质。有污垢的磁带驱动器磁头或旧的介质可能会导致很高的错误率，并相应地降低读／写速度。每次驱动器尝试在磁带上重写或重读区块时，性能都会下降。一旦到达了读／写错误的某个阈值，驱动器通常会要求清洗磁头。定期或在请求时清洗驱动器磁头是十分必要的。

当介质老化或过度使用时，区块损坏的机率就会增加。一片

次完全磁带读／写。

LTO

介质的典型寿命大约为

速率匹配。更高级的压缩数据传输速度的一半左右。如果向驱动器提供数据的速度低于速度匹配的下限，驱动器必须停止传输，等待缓冲区填充，重绕磁带，然后尝试写入缓冲区（此过程称为 “后系留”）。

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例如，

Dell PowerVault 110T （LT O 2 和 LTO3

速率匹配为最低区填充时，驱动器将出现“后系留”现象。在这种情况下，有效的吞吐量将低于（可能接近

确定磁带驱动器的性能

15 MB

某些磁带驱动器制造商在驱动器中内置了性能诊断模式，用于确定吞吐量。

和

LT O -2

LTO -3

执行快速读／写性能测试。如果性能速率超出了指定的最大驱动器速度的百分之六，则测试失败，并显示一条错误信息。如果测试通过了，则不会显示错误信息。有关诊断模式“

的详细信息，请参阅磁带驱动器的用户指南。

注意：作为诊断测试的一部分，诊断模式 "F" 要求可以安全地覆盖介质。不要使用包含重要数

据的介质。用于诊断测试的介质上包含的任何数据都将丢失。

硬盘驱动器和

许多硬盘驱动器和磁盘阵列（包括内部和外部）属性可能会影响备份或恢复性能。以下各小节将分别介绍这些属性以及建议采用的可帮助获得最大备份和恢复速度的配置。如果磁带驱动器的可承受吞吐量超过了磁盘阵列的吞吐量，则磁带驱动器将无法达到最佳性能。

30 MB

／秒）。

（固件

RAID

LTO

驱动器将与向驱动器提供的数据的速度相匹配，最低速率为最大未

）磁带驱动器在向

／秒。如果主机服务器仅以

20 MB

LTO -3

介质写入数据时，将

／秒的速率提供数据，在等待缓冲

PowerVault 110T

53XX

或更高版本）提供了一个诊断模式

"F"

，可以在驱动器和介质上

阵列配置

20 MB

／秒

”

硬盘驱动器配置一般注意事项

不同

LUN

上的数据／操作系统

(OS)

据，可确保硬盘驱动器不会在操作系统操作与备份操作之间拆分访问量和系统开销。为此，可通过使用一个硬盘驱动器或磁盘阵列包含操作系统，而使用一个物理上独立的硬盘驱动器或磁盘阵列来包含要备份的数据来实现这一要求。

30 磁带驱动器和磁带存储库性能注意事项

。在与操作系统

LUN

不同的逻辑设备号

(LUN)

上备份数

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