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GEOM_ZERO(4)		     Kernel Interfaces Manual		    GEOM_ZERO(4)

NAME
     gzero, geom_zero -- GEOM-based zero disk/block device

SYNOPSIS
     options GEOM_ZERO

     In loader.conf(5) or sysctl.conf(5):
     kern.geom.zero.byte
     kern.geom.zero.clear

DESCRIPTION
     gzero  is	a  GEOM(4) device simulating a one-exabyte disk.  It throws away
     any data written to it, and returns the value  of	kern.geom.zero.byte  for
     every byte read from it.

     gzero  differs from zero(4), which is a regular character device and has an
     infinite length, while /dev/gzero is a GEOM(4) provider of large, but  lim-
     ited, size.

     Consult  geom(8)  for  instructions on how to use the supported commands of
     the GEOM(4) ZERO class.

     gzero is useful for benchmarking performance of GEOM and GEOM classes where
     compression of the data does not affect the results (blocks from /dev/gzero
     compress exceptionally well).  Examples of such benchmarks include  compar-
     ing  the  speed  of two disk encryption algorithms and comparing a hardware
     versus software implementation of a single encryption algorithm.

MIB VARIABLES
     The following variables are  available  as  both  sysctl(8)  variables  and
     loader(8) tunables:

     kern.geom.zero.byte   This variable sets the fill byte of the gzero device.
			   Default: `0'.

     kern.geom.zero.clear  This  variable controls the clearing of the read data
			   buffer.  If set to `0', gzero will not copy any  data
			   into  the  read data buffers and just return the read
			   data buffers as they are without modifying them.   In
			   particular, it will not not fill the read buffer with
			   the value of kern.geom.zero.byte.  This is useful for
			   read  benchmarking  to  reduce  the measurement noise
			   caused by extra memory initialization.  Default: `1'.

FILES
     /dev/gzero  The gzero device.

EXAMPLES
     Create the /dev/gzero device by loading the geom_zero kernel module:

	   # geom zero load

     Show information about the gzero device:

	   # geom zero list
	   Geom name: gzero
	   Providers:
	   1. Name: gzero
	      Mediasize: 1152921504606846976 (1.0E)
	      Sectorsize: 512
	      Mode: r0w0egzero0

     Set the fill byte of the gzero device to 70  (decimal  for  letter  "F"  in
     ascii(7)):

	   # sysctl kern.geom.zero.byte=70
	   kern.geom.zero.byte: 0 -> 70
	   # head -c 1 /dev/gzero
	   F

     Benchmark	read  and write throughput of geli(8)'s default encryption algo-
     rithm with a 4-KiB sector size:

	   # geom zero load
	   # geli onetime -s 4096 gzero
	   # sysctl kern.geom.zero.clear=0
	   # dd if=/dev/gzero.eli of=/dev/zero bs=4k count=$((1024 * 256))
	   262144+0 records in
	   262144+0 records out
	   1073741824 bytes transferred in 1.258195 secs (853398307 bytes/sec)
	   # dd if=/dev/zero of=/dev/gzero.eli bs=4k count=$((1024 * 256))
	   262144+0 records in
	   262144+0 records out
	   1073741824 bytes transferred in 1.663118 secs (645619658 bytes/sec)

SEE ALSO
     GEOM(4), zero(4), geom(8), sysctl(8), bio(9)

HISTORY
     A gzero device first appeared in FreeBSD 6.

AUTHORS
     The gzero device was written by PaweA Jakub Dawidek <pjd@FreeBSD.org>.

     The  gzero  manual  page  was  originally	written  by  Greg   White   <gk-
     white@gmail.com>  and rewritten by Mateusz Piotrowski <0mp@FreeBSD.org> be-
     fore landing in FreeBSD.

FreeBSD ports 15.quarterly	November 9, 2025		    GEOM_ZERO(4)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=gzero&sektion=4&manpath=FreeBSD+15.1-RELEASE+and+Ports.quarterly>

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