14.1.1 Amount of Cache

It is possible to tune the following items:

Amount of free-extent based management information of a file system to be cached in memory
(SFCFS_NCACHE_EXTENT)
Amount of directory information to be cached in memory
(SFCFS_NCACHE_DIRECTORY)
Amount of i-nodes on the disk to be cached in memory
(SFCFS_NCACHE_INODE)

The areas of the following sizes are reserved in the process space of MDS for each file system:

Cache of free-extent based management information of a file system
Specified value of SFCFS_NCACHE_EXTENT x 1.4 kilobytes
Cache of directory information
Specified value of SFCFS_NCACHE_DIRECTORY x 1.4 kilobytes
Cache of i-node
Specified value of SFCFS_NCACHE_INODE x 4.4 kilobytes

The following shows the default value and amount of reserved memory for each tuning parameter.

Table 14.1 Default value and amount of reserved memory for tuning parameters
Tuning parameter name	Default value	Amount of reserved memory (megabyte)
SFCFS_NCACHE_EXTENT	2304	3.2
SFCFS_NCACHE_DIRECTORY	10240	14.0
SFCFS_NCACHE_INODE	2048	8.8

Specify the tuning parameters with the -o option of sfcadm(8).

The following shows the setting example.

(Example)

# sfcadm -o SFCFS_NCACHE_EXTENT=4096 /dev/sfdsk/gfs01/dsk/volume01 <Enter>
# sfcadm -o SFCFS_NCACHE_DIRECTORY=20480 /dev/sfdsk/gfs01/dsk/volume01 <Enter>
# sfcadm -o SFCFS_NCACHE_INODE=5120 /dev/sfdsk/gfs01/dsk/volume01 <Enter>

Check the specified values with the -e option of sfcinfo(8).

The following shows the example to check the values.

(Example)

# sfcinfo -e /dev/sfdsk/gfs01/dsk/volume01 | grep SFCFS_NCACHE <Enter>
SFCFS_NCACHE_EXTENT=4096
SFCFS_NCACHE_DIRECTORY=20480
SFCFS_NCACHE_INODE=5120

If the size of a directory exceeds the directory cache's size, performance of file creation in the directory or reference will fall. When a large directory is created, we recommend strongly that users specify a larger value for SFCFS_NCACHE_DIRECTORY.

Also, in the case of tuning, execute sfcstat(8) that reports statistics on GFS Shared File System by specifying the -m option, and check the cache-hit ratio of the meta-cache which corresponds to the tuning parameter. The cache-hit ratio is calculated from the number of accesses and the number of cache hits. If the cache-hit ratio is low, consider to specify a larger value of the tuning parameter.

The following shows the correspondence of each tuning parameter and meta-cache type displayed by the -m option of sfcstat(8).

Table 14.2 Correspondence of tuning parameter and meta-cache type displayed by -m option of sfcstat(8)
Tuning parameter name	Meta-cache type
SFCFS_NCACHE_EXTENT	NODE, LEAF
SFCFS_NCACHE_DIRECTORY	DIR
SFCFS_NCACHE_INODE	DINODE