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VNDCOMPRESS(1) General Commands Manual VNDCOMPRESS(1)
NAME
vndcompress, vnduncompress - compress and uncompress disk images in
cloop2 format
SYNOPSIS
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
DESCRIPTION
The 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 vndconfig(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 blocksize
Set the compression block size to blocksize, which must be a
multiple of 512 and must be no more than 4294966784, or
0xfffffe00. (On 32-bit systems, the limit may be smaller,
limited by the available virtual address space.)
For compatibility with the old version of vndcompress, the
compression block size may instead be specified at the end of the
command line.
-k checkpoint-blocks
Write a checkpoint after every checkpoint-blocks blocks of
output. If interrupted, vndcompress 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
Specify the length of the input, so that the input may be a pipe,
socket, or FIFO. Otherwise, the input must know its size, i.e.
must have its size reported in st_size by fstat(2).
-p partial-blocks
Stop after writing partial-blocks blocks of output. This option
is mainly useful for automatic testing of restarts.
-R With the -r option, if restarting fails, then abort right now and
don't touch the output file.
-r Try to restart a partial compression from the last checkpoint.
If restarting fails, and the -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 winsize
Use an in-memory window of winsize entries into the table of
compressed block offsets. If winsize is zero, vndcompress 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.
EXIT STATUS
The vndcompress utility exits 0 on success, and >0 if an error occurs.
EXAMPLES
Compress an ISO 9660 CD-ROM image:
# 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:
# vndconfig -z vnd0 disk.cloop2
# mount /dev/vnd0d /mnt
SIGNALS
vndcompress responds to the following signals:
SIGINFO, SIGUSR1
Report progress to standard error.
SIGUSR2
Write a checkpoint to disk now.
FORMAT
The cloop2 format consists of a header, an offset table, and a sequence
of compressed blocks. The header is described by the following packed
structure:
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.
SEE ALSO
vnd(4), vndconfig(8)
HISTORY
The 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.
NetBSD 10.99 January 24, 2020 NetBSD 10.99