分类：
2008-09-06 13:42:31
介绍如何分析磁带机带库备份性能的问题：
Reasons why the data throughput rate can be slower than the theoretical maximum when backing up to tape media.
Details:
Backup performance varies between similar systems, and the backup performance of any system is directly related to one or more of the following performance factors:
a. Hardware
The speed of the disk controller and hardware errors caused by the disk drive, the tape drive, the disk controller, the SCSI bus, or the improper cabling/termination can slow performance.
Confirm that the controller is rated for the tape backup hardware and that the SCSI Bios Settings are set properly. Newer models of SCSI Controllers are set to communicate with SCSI Hard Drives by default. Most tape drives can only handle a maximum sync transfer rate (bus speed) of between 3 to 22 MB/sec when utilizing hardware compression. Speed in excess of this will not only affect the ability for data to write to the tape in a continuous stream, but can also potentially damage the tape hardware.
Enable disconnect and enable Sync Negotiation in most cases should be set to NO.
Initiate Wide Negotiation should only be set to Yes when the tape device is connected to a 68 pin wide SCSI Cable Connector
A tape drive should never be connected to a SCSI Raid Controller.
b. System
Disk/File Fragmentation — Fragmented disks take a longer time to back up. Heavily fragmented hard disks not only affect the rate at which data is written to tape, but also affects the overall system performance. Fragmented files take longer to back up because each segment of data is located at a different location on the disk so instead of simply reading block after block of data, the heads on the disk must travel to a number of places to access the data. Performance can usually be improved by performing regular disk defragmentation.
System activity during backup will also slow performance on your tape drive.
c. Available Memory
The amount of available memory will impact backup speed. Insufficient memory, improper page file settings, and a lack of available free hard disk space will cause excessive paging and slow performance.
d. File Types
The average file can potentially compress at a 2:1 ratio when hardware compression is used. Higher and lower compression will occur depending on the type of files being backed up. Average compression can double the backup speed, while no compression simply runs the tape device at its rated speed.
Image and picture files are fully compressed on disks. Therefore, when backing up these type of files, no hardware compression takes place, which means that the tape drive is operating at its native (non-compression) rate of speed. Hardware compression is performed by the tape device and not the backup software.
e. Compression
Successful compression can increase the tape drive's data transfer rate up to twice the native rate. Some tape drives use the Lempel-Ziv (LZ1) compression algorithm for its superior versatility and efficiency. Compression can be highly variable depending on your input data. Compression algorithms look for repeatable data patterns that can be compacted.
Image files from a graphical program like Microsoft Paint, may compress at 4.5:1 or more, while binary files may compress at just 1.5:1. Data that has already been compressed or random data (such as encrypted data or MPEG files) may actually expand by about five percent if you attempt to compress it further. This can reduce drive throughput.
f. Size and Number of Files
The total number of files on a disk and the relative size of each file can either speed up backup or slow it down. Fastest backups occur when the disk contains a few large size files. Slowest backups occur when the disk contains thousands of small files. A large number of files located in the same directory path will back up more efficiently compared to backing them up from multiple locations.
g. Block Size
The bigger the block sizes the better the compression ratio. With better compression ratio, the drive is able to achieve not only better throughput, but also increased capacity on the tape. Confirm that the block and buffer size are set properly. The throughput will increase in proportion to the compression achieved, until the drive's maximum throughput is reached. VERITAS does not recommend increasing the Block Size above the default settings.
h. Network
The backup speed for a remote disk is limited by the speed of the physical connection. The rate at which a remote server's hard disks are able to be backed up at depends on the make/model of network cards, the mode/frame type configuration for the adapter, the connectivity equipment (hubs, switches, routers, and so on), and the Windows NT 4 or Windows 2000 Settings. Local disk drives on the Backup Exec Server can usually be backed up at a higher rate of speed than backing up remote servers across a network.
