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GCACHE(8)		FreeBSD	System Manager's Manual		     GCACHE(8)

     gcache -- control utility for CACHE GEOM class

     gcache create [-v]	[-b blocksize] [-s size] name prov
     gcache configure [-v] [-b blocksize] [-s size] name
     gcache destroy [-fv] name
     gcache label [-v] [-b blocksize] [-s size]	name prov
     gcache stop [-fv] name ...
     gcache clear [-v] prov ...
     gcache dump prov ...
     gcache list
     gcache status [-s name]
     gcache load [-v]
     gcache unload [-v]

     The gcache	utility	is used	to control GEOM	cache, which can speed up read
     performance by sending fixed size read requests to	its consumer.  It has
     been developed to address the problem of a	horrible read performance of a
     64k blocksize FS residing on a RAID3 array	with 8 data components,	where
     a single disk component would only	get 8k read requests, thus effectively
     killing disk performance under high load.

     Caching can be configured using two different methods: ``manual'' or
     ``automatic''.  When using	the ``manual'' method, no metadata are stored
     on	the devices, so	the cached device has to be configured by hand every
     time it is	needed.	 The ``automatic'' method uses on-disk metadata	to
     detect devices.  Once devices are labeled,	they will be automatically
     detected and configured.

     The first argument	to gcache indicates an action to be performed:

     create   Cache the	given devices with specified name.  This is the
	      ``manual'' method.  The kernel module geom_cache.ko will be
	      loaded if	it is not loaded already.

     label    Cache the	given devices with the specified name.	This is	the
	      ``automatic'' method, where metadata are stored in every
	      device's last sector.  The kernel	module geom_cache.ko will be
	      loaded if	it is not loaded already.

     stop     Turn off existing	cache device by	its name.  This	command	does
	      not touch	on-disk	metadata!

     destroy  Same as stop.

     clear    Clear metadata on	the given devices.

     dump     Dump metadata stored on the given	devices.

     list     See geom(8).

     status   See geom(8).

     load     See geom(8).

     unload   See geom(8).

     Additional	options:

     -f	     Force the removal of the specified	cache device.

     -v	     Be	more verbose.

     The following sysctl(8) variables can be used to control the behavior of
     the CACHE GEOM class.  The	default	value is shown next to each variable.

     kern.geom.cache.used_hi: 20

     kern.geom.cache.used_lo: 5

     kern.geom.cache.idletime: 5

     kern.geom.cache.timeout: 10

     kern.geom.cache.enable: 1

     kern.geom.cache.debug: 0
	     Debug level of the	CACHE GEOM class.  This	can be set to a	number
	     between 0 and 3 inclusive.	 If set	to 0 minimal debug information
	     is	printed, and if	set to 3 the maximum amount of debug informa-
	     tion is printed.

     Exit status is 0 on success, and 1	if the command fails.

     geom(4), geom(8)

     The gcache	utility	appeared in FreeBSD 7.0.

     Ruslan Ermilov <>

FreeBSD	Ports 11.2		January	3, 2010		    FreeBSD Ports 11.2


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