[Libguestfs] [PATCH libnbd] PROPOSAL Add nbdcp (NBD copying) tool.

[Richard W.M. Jones]

This is a proposal for an NBD to/from file copying tool (not actually
written).  Obviously this would duplicate functionality which is
already available in qemu-img convert.

The reasons for writing this tool would be:

 - to produce a tool which is very focused on the specific needs of
   virt-v2v and similar migration scenarios

 - to have a small tool with minimal dependencies

 - fix some of the obvious problems with qemu-img convert like how it
   handles devices which are already zeroed, and preallocation

 - enable optimizations which make sense for NBD but which might not
   be applicable to qemu (eg. multi-conn, freely writing out of order
   from multiple threads).

Alternative is of course to not write a new tool and instead try to
fix all this stuff in qemu-img itself.

Anyway let me know your thoughts.

Rich.


nbdcp(1)                            LIBNBD                            nbdcp(1)

NAME
       nbdcp - copy between NBD servers and local files

SYNOPSIS
        nbdcp [-a|--target-allocation allocated|sparse]
              [-m|--multi-conn <n>] [-M|--multi-conn-target <n>]
              [-p|--progress-bar] [-S|--sparse-detect <n>]
              [-T|--threads <n>] [-z|--target-is-zero]
              'nbd://...'|DISK.IMG 'nbd://...'|DISK.IMG

DESCRIPTION
       nbdcp is a utility that can copy quickly between NBD servers and local
       raw format files (or block devices).  It can copy:

       from NBD server to file (or block device)
           For example, this command copies from the NBD server listening on
           port 10809 on "example.com" to a local file called disk.img:

            nbdcp nbd://example.com disk.img

       from file (or block device) to NBD server
           For example, this command copies from a local block device /dev/sda
           to the NBD server listening on Unix domain socket /tmp/socket:

            nbdcp /dev/sda 'nbd+unix:///?socket=/tmp/socket'

       from NBD server to NBD server
           For example this copies between two different exports on the same
           NBD server:

            nbdcp nbd://example.com/export1 nbd://example.com/export2

       This program cannot: copy from file to file (use cp(1) or dd(1)), copy
       to or from formats other than raw (use qemu-img(1) convert), or access
       servers other than NBD servers (also use qemu-img(1)).

       NBD servers are specified by their URI, following the NBD URI standard
       at https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md

   Controlling sparseness or preallocation in the target
       The options -a (--target-allocation), -S (--sparse-detect) and -z
       (--target-is-zero) together control sparseness in the target file.

       By default nbdcp tries to both preserve sparseness from the source and
       will detect runs of allocated zeroes and turn them into sparseness.  To
       turn off detection of sparseness use "-S 0".

       The -z option should be used if and only if you know that the target
       block device is zeroed already.  This allows an important optimization
       where nbdcp can skip zeroing or trimming parts of the disk that are
       already zero.

       The -a option is used to control the desired final preallocation state
       of the target.  The default is "-a sparse" which makes the target as
       sparse as possible.  "-a allocated" makes the target fully allocated.

OPTIONS
       --help
           Display brief command line help and exit.

       -a allocated
       --target-allocation=allocated
           Make the target fully allocated.

       -a sparse
       --target-allocation=sparse
           Make the target as sparse as possible.  This is the default.  See
           also "Controlling sparseness or preallocation in the target".

       -m N
       --multi-conn=N
           Enable NBD multi-conn with up to "N" connections.  Only some NBD
           servers support this but it can greatly improve performance.

           The default is to enable multi-conn if we detect that the server
           supports it, with up to 4 connections.

       -M N
       --multi-conn-target=N
           If you are copying between NBD servers, use -m to control the
           multi-conn setting for the source server, and this option (-M) to
           control the multi-conn setting for the target server.

       -p
       --progress-bar
           Display a progress bar during copying.

       -p machine:FD
       --progress-bar=machine:FD
           Write a machine-readable progress bar to file descriptor "FD".
           This progress bar prints lines with the format "COPIED/TOTAL"
           (where "COPIED" and "TOTAL" are 64 bit unsigned integers).

       -S 0
       --sparse-detect=0
           Turn off sparseness detection.

       -S N
       --sparse-detect=N
           Detect runs of zero bytes of at least size "N" bytes and turn them
           into sparse blocks on the target (if "-a sparse" is used).  This is
           the default, with a 512 byte block size.

       -T N
       --threads N
           Use at most "N" threads when copying.  Usually more threads leads
           to better performance, up to the limit of the number of cores on
           your machine and the parallelism of the underlying disk or network.
           The default is to use the number of online processors.

       -z
       --target-is-zero
           Declare that the target block device contains only zero bytes (or
           sparseness that reads back as zeroes).  You must only use this
           option if you are sure that this is true, since it means that nbdcp
           will enable an optimization where it skips zeroing parts of the
           disk that are zero on the source.

       -V
       --version
           Display the package name and version and exit.

SEE ALSO
       qemu-img(1), libnbd(3), nbdsh(1).

AUTHORS
       Richard W.M. Jones

COPYRIGHT
       Copyright (C) 2020 Red Hat Inc.

LICENSE
       This library is free software; you can redistribute it and/or modify it
       under the terms of the GNU Lesser General Public License as published
       by the Free Software Foundation; either version 2 of the License, or
       (at your option) any later version.

       This library is distributed in the hope that it will be useful, but
       WITHOUT ANY WARRANTY; without even the implied warranty of
       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
       Lesser General Public License for more details.

       You should have received a copy of the GNU Lesser General Public
       License along with this library; if not, write to the Free Software
       Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
       02110-1301 USA

libnbd-1.3.1                      2020-01-22                          nbdcp(1)