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geom_uncompress(4) [freebsd man page]

GEOM_UNCOMPRESS(4)					   BSD Kernel Interfaces Manual 					GEOM_UNCOMPRESS(4)

NAME
geom_uncompress -- GEOM based compressed disk images SYNOPSIS
To compile this driver into the kernel, place the following line in your kernel configuration file: options GEOM_UNCOMPRESS Alternatively, to load the driver as a module at boot time, place the following line in loader.conf(5): geom_uncompress_load="YES" DESCRIPTION
The geom_uncompress framework provides support for compressed read only disk images. This allows significant storage savings at the expense of a little CPU time on each read. Data written in the GEOM label area allows geom_uncompress to detect compressed images which have been created with mkulzma(8) or mkuzip(8) and presented to the kernel as a logical disk device via md(4). geom_uncompress creates a unique md#.uncompress device for each image. The geom_uncompress device is subsequently used by the FreeBSD kernel to access the disk images. The geom_uncompress driver does not allow write operations to the underlying disk image. To check which md(4) devices match a given geom_uncompress device: # geom uncompress list Geom name: md0.uncompress Providers: 1. Name: md0.uncompress Mediasize: 52428800 (50M) Sectorsize: 512 Mode: r1w0e0 Consumers: 1. Name: md0 Mediasize: 20864000 (20M) Sectorsize: 512 Mode: r1w0e0 SEE ALSO
GEOM(4), md(4), geom(8), mkulzma(8), mkuzip(8) AUTHORS
The geom_uncompress driver was written by Maxim Sobolev <sobomax@FreeBSD.org> and Aleksandr Rybalko <ray@FreeBSD.org>. This manual page was written by Luiz Otavio O Souza <loos@FreeBSD.org>. BSD
January 9, 2014 BSD

Check Out this Related Man Page

MKULZMA(8)						    BSD System Manager's Manual 						MKULZMA(8)

NAME
mkulzma -- compress disk image for use with geom_uncompress(4) class SYNOPSIS
mkulzma [-v] [-o outfile] [-s cluster_size] infile DESCRIPTION
The mkulzma utility compresses a disk image file so that the geom_uncompress(4) class will be able to decompress the resulting image at run- time. This allows for a significant reduction of size of disk image at the expense of some CPU time required to decompress the data each time it is read. The mkulzma utility works in two phases: 1. An infile image is split into clusters; each cluster is compressed using liblzma. 2. The resulting set of compressed clusters along with headers that allow locating each individual cluster is written to the output file. The options are: -o outfile Name of the output file outfile. The default is to use the input name with the suffix .ulzma. -s cluster_size Split the image into clusters of cluster_size bytes, 16384 bytes by default. The cluster_size should be a multiple of 512 bytes. -v Display verbose messages. NOTES
The compression ratio largely depends on the cluster size used. For large cluster sizes (16K and higher), typical compression ratios are only 1-2% less than those achieved with lzma(1). However, it should be kept in mind that larger cluster sizes lead to higher overhead in the geom_uncompress(4) class, as the class has to decompress the whole cluster even if only a few bytes from that cluster have to be read. The mkulzma utility inserts a short shell script at the beginning of the generated image, which makes it possible to ``run'' the image just like any other shell script. The script tries to load the geom_uncompress(4) class if it is not loaded, configure the image as an md(4) disk device using mdconfig(8), and automatically mount it using mount_cd9660(8) on the mount point provided as the first argument to the script. EXIT STATUS
The mkulzma utility exits 0 on success, and >0 if an error occurs. SEE ALSO
lzma(1), geom(4), geom_uncompress(4), md(4), mdconfig(8), mount_cd9660(8) AUTHORS
Maxim Sobolev <sobomax@FreeBSD.org> Aleksandr Rybalko <ray@ddteam.net> BSD
March 17, 2006 BSD

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