Chapter 8: Statistics
Statistic | Description |
|
|
Access Size | This is the percentage of commands of the size |
| specified. On occasions, an application may specify a |
| particular access size, but the operating system can |
| reduce this. This can lead to performance problems. For |
| example, on Windows NT, Windows 2000, and |
| Windows 2003 the maximum command size that the |
| operating system will allow is 1MB. Additionally, some |
| HBAs further reduce this value by default. However, |
| some benchmark programs allow a larger access size |
| to be specified, resulting in misleading results. By |
| viewing the statistics on the controller, it is possible to |
| determine the exact size of commands sent by the host, |
| and so determine whether the results are appropriate. |
| The access size, in conjunction with the alignment, |
| gives an indication of how many drives are involved in |
| an access. For example, consider a RAID 5/50 array |
| with a chunk size of 64 K. In this case, a 64 K access |
| with an alignment of 8 K will actually involve 2 data |
| drives, since it needs to access some data in the first |
| drive, and the remaining data in the next drive. This is |
| clearly inefficient, and could be improved by setting the |
| alignment to 64 K on the operating system. If that is not |
| possible, using a larger chunk size can help, as this |
| reduces the number of accesses that span chunks. |
| Aligning an access on the same value as the access |
| size will improve performance, as it will ensure that |
| there are not |
| are smaller than a chunk size. |
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