IBM AS/400E manual Directory and File Attribute Cache, Data cache

Client-side caching in NFS reduces the number of RPC requests sent to the server.

The NFS client can cache data, which can be read out of local memory instead of

from a remote disk. The caching scheme available for use depends on the ®le

system being accessed. Some caching schemes are prohibited because they

cannot guarantee the integrity and consistency of data that multiple clients

simultaneously change and update. The standard NFS cache policies ensure that

performance is acceptable while also preventing the introduction of state into the

client/server communication relationship.

There are two types of client caches: the directory and ®le attribute cache and

the data cache.

Directory and File Attribute Cache

Not all ®le system operations use the data in ®les and directories. Many operations

get or set the attributes of the ®le or directory, such as its length, owner, and

modi®cation time. Because these attribute-only operations are frequent and do not

affect the data in a ®le or directory, they are prime candidates for using cached

information.

The client-side ®le and directory cache will store ®le attributes. The system does

this so that every operation that gets or sets attributes does not have to go through

the connection to the NFS server. When the system reads a ®le's attributes, they

remain valid on the client for some minimum period of time, typically 30 seconds.

You can set this time period by using the acregmin option on the mount command.

If the client changes the ®le, he updates its attributes. This makes changes to the

local copy of the attributes and extends the cache validity period another minimum

time period. The attributes of a ®le remain static for some maximum period, typically

sixty seconds. Additionally, the system deletes ®le attributes from the cache, and

writes changed ®le attributes back to the server. You can set this time period with

the acregmax option on the mount command. To force a refresh of remote attributes

when opening a ®le, use thenocto option on the mount command. Specifying the

noac option suppresses all the local caching of attributes, negating the acregmin,

acregmax, acdirmin, and the acdirmax options on the mount command.

The same mechanism is used for directory attributes, although they are given a

longer minimum life-span. The minimum and maximum time period for directory

attribute ¯ushing from the cache is set by theacdirmin and acdirmax options on the

mount command.

Attribute caching allows a client to make multiple changes to a ®le or directory

without having to constantly get and set attributes on the server. Intermediate

attributes are cached, and the sum total of all updates is later written to the server

when the maximum attribute cache period expires. Frequently accessed ®les and

directories have their attributes cached locally on the client so that some NFS

requests can be performed without having to make an RPC call. By preventing this

type of client/server interaction, caching attributes improves the performance of

NFS.

For more information on the ADDMFS and MOUNT commands, see ªChapter 5.

Client Mounting of File Systemsº on page 39. For more information on the options

to the ADDMFS and MOUNT commands, see CL Reference, SC41-4722.

Chapter 2. The Network File System Client/Server Model 13