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GEOM_UZIP(4) Device Drivers Manual GEOM_UZIP(4)


geom_uzipGEOM based compressed disk images and partitions


To compile this driver into the kernel, place the following line in your kernel configuration file:

device xz
options GEOM_UZIP

Alternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):



The geom_uzip framework provides support for compressed read-only disk images. This allows significant storage savings at the expense of a some CPU time on each read. Data written in the GEOM label area allows geom_uzip to detect compressed images which have been created with mkuzip(8) and presented to the kernel as a logical disk device via md(4). geom_uzip creates a unique md#.uzip device for each image.

geom_uzip is not limited to supporting only md(4) images. The image can also reside on a block device. (For example, a disk, USB flash drive, DVD-ROM, etc.) The appropriate device node will appear with the .uzip suffix.

# gpart show da0
=>      0  7833600  da0  BSD  (3.7G)
        0  2097152    1  freebsd-ufs  (1.0G)
  2097152  5736448       - free -  (2.7G)
# gpart add -t freebsd-ufs -s 1G da0
da0b added
# dd if=/tmp/20160217_dcomp_zcomp.uzip bs=256k of=/dev/da0b
3190+1 records in
3190+1 records out
836331008 bytes transferred in 111.021489 secs (7533055 bytes/sec)
# fsck -t ffs /dev/da0b.uzip
** /dev/da0b.uzip (NO WRITE)
** Last Mounted on /mnt
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
97455 files, 604242 used, 184741 free (2349 frags, 22799 blocks,
   0.3% fragmentation)
# mount -o ro /dev/da0b.uzip /mnt
# df /dev/da0b.uzip
Filesystem     1K-blocks    Used  Avail Capacity  Mounted on
/dev/da0b.uzip   3155932 2416968 738964    77%    /mnt

The geom_uzip device is subsequently used by FreeBSD kernel to access the uncompressed data. The geom_uzip driver does not allow write operations to the underlying disk image. To check which “providers” match a given geom_uzip device:

# geom uzip list
Geom name: md1.uzip
1. Name: md1.uzip
   Mediasize: 22003712 (21M)
   Sectorsize: 512
1. Name: md1
   Mediasize: 9563648 (9.1M)
   Sectorsize: 512

Geom name: da0b.uzip
1. Name: da0b.uzip
   Mediasize: 3355443200 (3.1G)
   Sectorsize: 512
1. Name: da0b
   Mediasize: 1073741824 (1.0G)
   Sectorsize: 512

geom_uzip allows mounting the root file system from a compressed disk partition by setting the vfs.root.mountfrom tunable. See loader.conf(5) for details.


Several flags are provided for tracing geom_uzip I/O operations and TOC parsing via the following sysctls.

Log level. Zero disables logging. Higher values enable more verbose debug logging for geom_uzip. Supported levels are from 0 (no logging) to 4 (maximum amount of logging).
Log operations involving compressed cluster number.


GEOM(4), md(4), geom(8), mkuzip(8)


The geom_uzip driver was written by Max Khon <>. The block de-duplication code as well as some geom_uzip driver optimizations have been contributed by Maxim Sobolev <>. The LZMA decompression support and CLOOP 3.0 support have been added by Aleksandr Rybalko <>.

This manual page was written by Ceri Davies <>.

February 26, 2019 Debian