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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 limited, 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 comparing the speed of two disk encryption algorithms and com- paring 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 de- vice. 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 re- turn 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 al- gorithm 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 Pawe Jakub Dawidek <pjd@FreeBSD.org>. The gzero manual page was originally written by Greg White <gkwhite@gmail.com> and rewritten by Mateusz Piotrowski <0mp@FreeBSD.org> before landing in FreeBSD. FreeBSD ports 15.quarterly November 9, 2025 GEOM_ZERO(4)
NAME | SYNOPSIS | DESCRIPTION | MIB VARIABLES | FILES | EXAMPLES | SEE ALSO | HISTORY | AUTHORS
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