Dell PowerVault TL4000 - Manual (Page 8)

8
Performance Considerations for Tape Drives and Libraries
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Random access time or seek time. Usually measured in milliseconds, seek time is the length of
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). STR measures how fast a drive actually reads data from
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
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). The buffer is the amount of memory on the drive that holds the most recently
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.