Measures reported by VnxCacheStatTest
Data movers (also known as X-Blades) contain the defined file system. They access data from the back end and provide host access using the same UltraFlex I/O Technology that supports the NFS, CFS, MPFS, and pNFS protocols. Each data mover has a large cache, which serves NFS I/O requests and reduces the I/O going into the storage processor. If users to VNX complain that their NFS requests take time being serviced, you may want to check cache usage to see if it has been used effectively or not. The VnxCacheStatTest test can be used to perform this periodic cache usage check. For each data mover in the DME of a VNX system, this test monitors the directory name lookup, openfile, and buffer caches, and reports the percentage of requests that have been serviced by each of these caches. Deficiencies in cache usage are thus revealed, thereby enabling administrators to assess its impact on storage system performance and to investigate the reasons for the bad cache usage.
The measures made by this test are as follows:
| Measurement |
Description |
Measurement Unit |
Interpretation |
| Lookup_hit_ratio |
Indicates the percentage of time the requested directory pathname was readily available in the Directory Name Lookup Cache (DNLC) without requiring to perform disk I/O on this data mover. |
Percent |
A high value of this measure (close to 80%) indicates that the Directory Name Lookup Cache is being utilized well. A smaller value is a cause for concern, as it not only indicates poor cache usage, but also implies that direct disk accesses are more. One of the key reasons for a low value for this measure is an improperly sized directory name lookup cache. If this is the case, then, you may want to increase the DNLC size to improve cache usage and reduce the I/O going straight to the disk. |
| File_hit_ratio |
Indicates the percentage of time the requested file is readily available in the open file cache without requiring to perform disk I/O on this data mover. |
Percent |
A high value of this measure (close to 80%) indicates that the open file cache is being utilized well. A smaller value is a cause for concern, as it not only indicates poor cache usage, but also implies that direct disk accesses are more. One of the key reasons for a low value for this measure is an improperly sized open file cache. If this is the case, then, you may want to increase the size of this cache to improve cache usage and reduce the I/O going straight to the disk. |
| Buffer_hit_ratio |
Indicates the percentage of requested pages that were readily available in the buffer memory of this data mover without performing disk I/O. |
Percent |
A high value of this measure (close to 80%) indicates that the kernel buffer cache is being utilized well. A smaller value is a cause for concern, as it not only indicates poor cache usage, but also implies that direct disk accesses are more. One of the key reasons for a low value for this measure is an improperly sized open file cache. If this is the case, then, you may want to increase the size of this cache to improve cache usage and reduce the I/O going straight to the disk. |
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