VNDCOMPRESS(1) | General Commands Manual | VNDCOMPRESS(1) |
vndcompress
, vnduncompress
—
vndcompress |
[-c ] [-rR ]
[-b blocksize]
[-k checkpoint-blocks]
[-l length]
[-p partial-offset]
[-w winsize]
image compressed-image
[blocksize] |
vnduncompress |
[-d ] [-w
winsize] compressed-image
image |
vndcompress
utility compresses disk images in cloop2
format, which the kernel's vnd(4)
device can interpret as read-only disk devices using the
-z
option to
vnconfig(8).
By default, vndcompress
compresses an
image, and vnduncompress
uncompresses an image, but
the -c
and -d
options can
control whether either utility compresses or decompresses.
The following options are available for the compression operation:
-b
blocksizeFor compatibility with the old version of
vndcompress
, the compression block size may
instead be specified at the end of the command line.
-k
checkpoint-blocksvndcompress
can
restart at the last checkpoint with the -r
option.
You can also request a checkpoint at any time by sending
SIGUSR2
to the vndcompress
process.-l
length-p
partial-blocks-R
-r
option, if restarting fails, then
abort right now and don't touch the output file.-r
-R
option is not
specified, then truncate the output file and start compression afresh.
Restarting may fail for various reasons: if there have been no
checkpoints, or if the output file has been corrupted in some easily
recognizable ways, or if the input file's size has changed, or if the
block size does not match, and so on.The following option is available for both compression and decompression:
-w
winsizevndcompress
will use memory proportional to
the number of blocks in the uncompressed image, namely 64 bits or 8 bytes
per block. If winsize is nonzero,
vndcompress
will use memory proportional to
winsize, and independent of the size of the
uncompressed image.
A nonzero winsize requires the compressed image to be a seekable file, which compression requires anyway, in order to record the offsets of compressed blocks once they are compressed and written, but which is a limitation for decompression. Thus, decompressing from a pipe is incompatible with a nonzero winsize.
By default, vndcompress
uses a fixed
window size, unless decompressing with nonseekable input.
vndcompress
utility exits 0 on success,
and >0 if an error occurs.
# vndcompress cdrom.iso cdrom.izo
Send a 59 GB disk partition over a local network with netcat (don't do this over the internet!):
# nc 10.0.1.2 12345 < /dev/rcgd1h
Receive it and save it compressed on another host, showing a progress bar interactively, restarting if possible, and taking a checkpoint every 16 MB (i.e., every 256 compression blocks, which are 64 KB by default):
# nc -l 12345 | progress -l 59g \ vndcompress -l 59g -k 256 -r /dev/stdin disk.cloop2
If the process is interrupted and you rewire your network and
disks so that the receiving host now has the disk you want to image, you can
start up where you left off, using the -R
option to
keep vndcompress
from clobbering the partial
transfer if anything goes wrong:
# vndcompress -l 59g -k 256 -rR /dev/rcgd1h disk.cloop2
Mount the disk with
vnd(4), assuming your kernel was
built with the VND_COMPRESSION
option enabled:
# vnconfig -z vnd0 disk.cloop2 # mount /dev/vnd0d /mnt
vndcompress
responds to the following signals:
struct cloop2_header { char cl2h_magic[128]; uint32_t cl2h_blocksize; uint32_t cl2h_n_blocks; } __packed;
The cl2h_magic field is an arbitrary sequence of 128 bits, the cl2h_blocksize field is a big-endian integer number of bytes per compression block, and the cl2h_n_blocks field is a big-endian integer number of the compression blocks in the image.
The offset table is a sequence of one more than cl2h_n_blocks big-endian 64-bit integers specifying the offset of each compression block relative to the start of the file. The extra offset specifies the end of the last compression block, which may be truncated if the uncompressed image's size is not a multiple of the compression block size.
vndcompress
command first appeared in
NetBSD 3.0. It was rewritten to be more robust, to
support restarting partial transfers, and to support bounded memory usage in
NetBSD 7.0.
January 21, 2014 | NetBSD 9.4 |