cachefs(4)cachefs(4)NAMEcachefs - Cache File System Description
DESCRIPTION
Cachefs is a file system layered above other standard IRIX file systems.
It performs automatic local caching of data from one file system
(typically remote) on a local file system. Any of the following file
systems may be used for the cached data: xfs and efs. Any of the
following file systems may be used as the source of data to be cached:
nfs, nfs3, iso9660, cdfs, hfs, kfs, dos, and udf.
Data is cached when it is first accessed. Subsequent accesses will
retrieve the data from the cache. Consistency will be maintained in a
manner similar to that used for NFS.
TERMINOLOGY
front file system
This is the local file system on which cached data is stored.
back file system
This is the file system which contains the definitive copy of the
data. Data is retrieved from this file system and cached on the
front file system.
cnode
This is the internal data structure used by cachefs to manage its
files. There is one cnode for each open file. Each cnode occupies
128 bytes on a 32 bit system and 256 bytes on a 64 bit system. In
addition, each cnode points to a dynamically allocated area of 512
bytes containing metadata and attributes.
TUNING
Cachefs has a number of different tuning parameters, some of which are
supplied on the mount(1M) command (see fstab(4)), some of which are
supplied via cfsadmin(1M), and some of which are kernel tuanbles
adjustable with systune(1M).
The kernel tunables are adjustable on a running kernel and control
readahead, asynchronous operations, and cnode caching. The kernel
tunables are described below.
cachefs_readahead
This controls the number of blocks to read ahead of the current
block being read. These will be read asynchronously. The size of
a block is whatever the preferred I/O size is for the front file
system.
cachefs_max_threads
This is the maximum number of asynchronous I/O daemons allowed to
be running per cachefs mounted file system.
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cachefs(4)cachefs(4)
fileheader_cache_size
This is the size in 512-byte units of the in-memory cache of file
header information (cached attributes and other metadata). This
number indicates the number of such headers retained after other
system internal data (e.g., the vnode) for a file has been
released.
replacement_timeout
This controls the time between reconstructions of the replacement
list by the replacement daemon (cachefs_replacement). If no
replacement requests are made by the kernel within this time
limit, the daemon will time out and reconstruct the list.
CONFIGURATION FILES
Mounts of cachefs file systems performed at system startup use options
supplied in /etc/config/cachefs.options. This file may contain any
options for mount(1M) desired. Of particular interest is the "-m"
option. This option allows the administrator to limit the number of
parallel mount processes mounting cachefs file systems. This will be
required on systems where some cachefs file systems fail to mount due to
a lack of swap space.
LIMITATIONS
Mandatory file locking is not supported on cachefs.
SEE ALSOcfsadmin(1M), fstab(4), systune(1M), mount(1M)
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