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WAFL is the NetApp® Write Anywhere File Layout, which defines how NetApp lays out data on disk. The WAFL buffer cache is a read cache maintained by WAFL in system memory. On a storage system, if you attempt to read data that is not in the WAFL buffer cache, it results in a direct disk read. Disk reads are expensive operations that increase the processing overheads of your storage system. A well-tuned, right-sized buffer cache can alone help in keeping disk reads minimal. By closely tracking the requests to the storage system and how the WAFL buffer cache services these requests, the NetApp File Layouts test points you to the ineffective usage (if any) of the buffer cache, which can be attributed to insufficient cache memory. Based on the findings of this test, you can then proceed to increase the cache memory (if required).

Measurement	Description	Measurement Unit	Interpretation
Name_cache_hit	Indicates the rate at which the name cache buffer was successfully queried for an entry during the last measurement period.	Hits/sec	While a high value is desired for the Name cache hits measure, a low value is ideal for Name cache misses. A large number of cache misses and very few cache hits indicate that adequate entries are not available in the cache to service requests to the storage system. This in turn will force direct disk reads, thereby increasing the processing overheads of the storage system. To minimize disk reads and maximize cache reads, you can increase the WAFL cache memory using WAFL extended cache and the Performance Acceleration Module (PAM) family. WAFL extended cache is a software component of Data ONTAP and requires a license. WAFL extended cache provides extra WAFL cache memory to improve the performance of the storage system by reducing the number of disk reads. Once the extended cache is enabled, you can cache the following in it: Cache normal user data blocks: If you cache normal user data blocks, the WAFL extended cache interprets this setting as the buffer cache policy of keep and saves normal user data blocks in the extended cache. Caching low-priority user data blocks: You can cache low-priority user data blocks that are not normally stored by WAFL extended cache. Low-priority blocks include blocks read in large sequential scans that are not normally reused, and blocks that have been written to the storage system through a network-attached storage (NAS) protocol such as Network File System (NFS). Caching low-priority user data blocks is useful if you have workloads that fit within WAFL extended cache memory and if the workloads consist of either write followed by read or large sequential reads. Caching only system metadata: If the working set of the storage system is very large, such as a large e-mail server, you can cache only system metadata in WAFL extended cache memory by turning off both normal user data block caching and low-priority user data block caching. Integrating FlexShare buffer cache policies with WAFL extended cache: For additional cache control, you can integrate FlexShare buffer cache policies with the WAFL extended cache options. Doing so allows you to set caching policies on specific volumes. You can choose to enable only the FlexShare buffer cache policies without enabling all other FlexShare options.
Name_cache_Misses	Indicates the rate at which the user query for an entry failed in the name cache buffer during the last measurement period.	Misses/sec
Dir_find_hit	Indicates the rate at which the directories were successfully found using the WAFL buffer during the last measurement period.	Hits/sec	A large number of cache misses and very few cache hits indicate that adequate entries are not available in the cache to service requests to the storage system. This in turn will force direct disk reads, thereby increasing the processing overheads of the storage system. To minimize disk reads and maximize cache reads, you can increase the WAFL cache memory using WAFL extended cache and the Performance Acceleration Module (PAM) family. WAFL extended cache is a software component of Data ONTAP and requires a license. WAFL extended cache provides extra WAFL cache memory to improve the performance of the storage system by reducing the number of disk reads. Once the extended cache is enabled, you can cache the following in it: Cache normal user data blocks: If you cache normal user data blocks, the WAFL extended cache interprets this setting as the buffer cache policy of keep and saves normal user data blocks in the extended cache. Caching low-priority user data blocks: You can cache low-priority user data blocks that are not normally stored by WAFL extended cache. Low-priority blocks include blocks read in large sequential scans that are not normally reused, and blocks that have been written to the storage system through a network-attached storage (NAS) protocol such as Network File System (NFS). Caching low-priority user data blocks is useful if you have workloads that fit within WAFL extended cache memory and if the workloads consist of either write followed by read or large sequential reads. Caching only system metadata: If the working set of the storage system is very large, such as a large e-mail server, you can cache only system metadata in WAFL extended cache memory by turning off both normal user data block caching and low-priority user data block caching. Integrating FlexShare buffer cache policies with WAFL extended cache: For additional cache control, you can integrate FlexShare buffer cache policies with the WAFL extended cache options. Doing so allows you to set caching policies on specific volumes. You can choose to enable only the FlexShare buffer cache policies without enabling all other FlexShare options.
Dir_find_misses	Indicates the rate at which the user request failed to find a directory using the WAFL buffer during the last measurement period.	Misses/sec
Buffer_hash_hit	Indicates the rate at which the hash queue of the WAFL buffer was successfully queried for an entry during the last measurement period.	Hits/sec	A large number of cache misses and very few cache hits indicate that adequate entries are not available in the cache to service requests to the storage system. This in turn will force direct disk reads, thereby increasing the processing overheads of the storage system. To minimize disk reads and maximize cache reads, you can increase the WAFL cache memory using WAFL extended cache and the Performance Acceleration Module (PAM) family. WAFL extended cache is a software component of Data ONTAP and requires a license. WAFL extended cache provides extra WAFL cache memory to improve the performance of the storage system by reducing the number of disk reads. Once the extended cache is enabled, you can cache the following in it: Cache normal user data blocks: If you cache normal user data blocks, the WAFL extended cache interprets this setting as the buffer cache policy of keep and saves normal user data blocks in the extended cache. Caching low-priority user data blocks: You can cache low-priority user data blocks that are not normally stored by WAFL extended cache. Low-priority blocks include blocks read in large sequential scans that are not normally reused, and blocks that have been written to the storage system through a network-attached storage (NAS) protocol such as Network File System (NFS). Caching low-priority user data blocks is useful if you have workloads that fit within WAFL extended cache memory and if the workloads consist of either write followed by read or large sequential reads. Caching only system metadata: If the working set of the storage system is very large, such as a large e-mail server, you can cache only system metadata in WAFL extended cache memory by turning off both normal user data block caching and low-priority user data block caching. Integrating FlexShare buffer cache policies with WAFL extended cache: For additional cache control, you can integrate FlexShare buffer cache policies with the WAFL extended cache options. Doing so allows you to set caching policies on specific volumes. You can choose to enable only the FlexShare buffer cache policies without enabling all other FlexShare options.
Buffer_hash_misses	Indicates the rate at which a the user request failed to find an entry in the hash queue of the WAFL buffer during the last measurement period.	Misses/sec
Inode_cache_hit	Indicates the rate at which the inode information of a file was successfully found using the WAFL buffer during the last measurement period.	Hits/sec	A large number of cache misses and very few cache hits indicate that adequate entries are not available in the cache to service requests to the storage system. This in turn will force direct disk reads, thereby increasing the processing overheads of the storage system. To minimize disk reads and maximize cache reads, you can increase the WAFL cache memory using WAFL extended cache and the Performance Acceleration Module (PAM) family. WAFL extended cache is a software component of Data ONTAP and requires a license. WAFL extended cache provides extra WAFL cache memory to improve the performance of the storage system by reducing the number of disk reads. Once the extended cache is enabled, you can cache the following in it: Cache normal user data blocks: If you cache normal user data blocks, the WAFL extended cache interprets this setting as the buffer cache policy of keep and saves normal user data blocks in the extended cache. Caching low-priority user data blocks: You can cache low-priority user data blocks that are not normally stored by WAFL extended cache. Low-priority blocks include blocks read in large sequential scans that are not normally reused, and blocks that have been written to the storage system through a network-attached storage (NAS) protocol such as Network File System (NFS). Caching low-priority user data blocks is useful if you have workloads that fit within WAFL extended cache memory and if the workloads consist of either write followed by read or large sequential reads. Caching only system metadata: If the working set of the storage system is very large, such as a large e-mail server, you can cache only system metadata in WAFL extended cache memory by turning off both normal user data block caching and low-priority user data block caching. Integrating FlexShare buffer cache policies with WAFL extended cache: For additional cache control, you can integrate FlexShare buffer cache policies with the WAFL extended cache options. Doing so allows you to set caching policies on specific volumes. You can choose to enable only the FlexShare buffer cache policies without enabling all other FlexShare options.
Inode_cache_misses	Indicates the rate at which the inode information of a file was not found in the WAFL buffer during the last measurement period.	Misses/sec
Buffer_cache_hit	Indicates the rate at which the WAFL buffer cache was successfully queried during the last measurement period.	Hits/sec	A large number of cache misses and very few cache hits indicate that adequate entries are not available in the cache to service requests to the storage system. This in turn will force direct disk reads, thereby increasing the processing overheads of the storage system. To minimize disk reads and maximize cache reads, you can increase the WAFL cache memory using WAFL extended cache and the Performance Acceleration Module (PAM) family. WAFL extended cache is a software component of Data ONTAP and requires a license. WAFL extended cache provides extra WAFL cache memory to improve the performance of the storage system by reducing the number of disk reads. Once the extended cache is enabled, you can cache the following in it: Cache normal user data blocks: If you cache normal user data blocks, the WAFL extended cache interprets this setting as the buffer cache policy of keep and saves normal user data blocks in the extended cache. Caching low-priority user data blocks: You can cache low-priority user data blocks that are not normally stored by WAFL extended cache. Low-priority blocks include blocks read in large sequential scans that are not normally reused, and blocks that have been written to the storage system through a network-attached storage (NAS) protocol such as Network File System (NFS). Caching low-priority user data blocks is useful if you have workloads that fit within WAFL extended cache memory and if the workloads consist of either write followed by read or large sequential reads. Caching only system metadata: If the working set of the storage system is very large, such as a large e-mail server, you can cache only system metadata in WAFL extended cache memory by turning off both normal user data block caching and low-priority user data block caching. Integrating FlexShare buffer cache policies with WAFL extended cache: For additional cache control, you can integrate FlexShare buffer cache policies with the WAFL extended cache options. Doing so allows you to set caching policies on specific volumes. You can choose to enable only the FlexShare buffer cache policies without enabling all other FlexShare options.
Buffer_cache_misses	Indicates the rate at which an entry was not found in the the WAFL buffer cache upon a user query during the last measurement period.	Misses/sec
Bufs_total	Indicates the total number of buffers in this storage system.	Number
Bufs_available	Indicates the number of available buffers in this storage system.	Number	A high value is desired for this measure.
Wafl_msg	Indicates the total number of WAFL messages in this storage system.	Number
Avg_msg_latency	Indicates the average time taken for the execution of the WAFL messages during the last measurement period.	Milliseconds	Ideally, the value of this measure should be low. A high value indicates a slowdown indicating a processing bottleneck.
Failures_allocate	Indicates the total number of times the WAFL buffer failed to allocate the extent messages.	Number	Ideally, the value of this measure should be 0. Too many failures may result in processing bottlenecks thus leading to the slowdown of the storage system.
Total_blocks_read	Indicates the total number of blocks read from the WAFL buffer cache.	Number
Total_blocks_write	Indicates the total number of blocks written to the WAFL buffer cache.	Number