Directory Name Lookup Cache (DNLC)

Size the directory name lookup cache (DNLC) to a default value using maxusers. A large cache size (ncsize) significantly increases the efficiency of NFS servers with multiple clients.

1. To show the DNLC hit rate (cache hits), type vmstat -s.

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   % vmstat -s
   ... lines omitted
   79062 total name lookups (cache hits 94%)
   16 toolong

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Directory names less than 30 characters long are cached and names that are too long to be cached are also reported. A cache miss means that a disk I/O may be needed to read the directory when traversing the path name components to get to a file. A hit rate of less than 90 percent requires attention.

Cache hit rates can significantly affect NFS performance. getattr, setattr, and lookup usually represent greater than 50 percent of all NFS calls. If the requested information isn't in cache, the request will generate a disk operation that results in a performance penalty as significant as that of a read or write request. The only limit to the size of the DNLC cache is available kernel memory.

If the hit rate (cache hits) is less than 90 percent and a problem does not exist with the number of longnames, tune the ncsize variable (see "To Reset ncsize,") which follows. The variable ncsize refers to the size of the DNLC in terms of the number of name and vnode translations that can be cached. Each DNLC entry uses about 50 bytes of extra kernel memory.

To Reset ncsize

1. Set ncsize in the /etc/system file to values higher than the default (based on maxusers.)

   As an initial guideline, since dedicated NFS servers do not need a lot of RAM, maxusers will be low and the DNLC will be small; double its size.

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   set ncsize=5000

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   The default value of ncsize is:

ncsize (name cache) = 17 * maxusers + 90

• For NFS server benchmarks, set it as high as 16000.

• For maxusers = 2048, set it at 34906.

1. Reboot the system.

See "Increasing the Inode Cache"" which follows.