项目作者: tarka

项目描述 :
An extended `cp`
高级语言: Rust
项目地址: git://github.com/tarka/xcp.git
创建时间: 2018-11-21T23:12:49Z
项目社区:https://github.com/tarka/xcp

开源协议:GNU General Public License v3.0

下载


xcp: An extended cp

xcp is a (partial) clone of the Unix cp command. It is not intended as a
full replacement, but as a companion utility with some more user-friendly
feedback and some optimisations that make sense under certain tasks (see
below).

Crates.io
Github Actions
CircleCI
Packaging status

Warning: xcp is currently beta-level software and almost certainly contains
bugs and unexpected or inconsistent behaviour. It probably shouldn’t be used for
anything critical yet.

Please note that there are some known issues with copying files from virtual
filesystems (e.g. /proc, /sys). See this LWN
article
for an overview of some of the
complexities of dealing with kernel-generated files. This is a common problem
with file utilities which rely on random access; for example rsync has the
same issue.

Installation

Cargo

xcp can be installed directly from crates.io with:

  1. cargo install xcp

Arch Linux

xcp is available on the Arch Linux User Repository. If you use an AUR helper, you can execute a command such as this:

  1. yay -S xcp

NetBSD

xcp is available on NetBSD from the official repositories. To install it, simply run:

  1. pkgin install xcp

Features and Anti-Features

Features

  • Displays a progress-bar, both for directory and single file copies. This can
    be disabled with --no-progress.
  • On Linux it uses copy_file_range call to copy files. This is the most
    efficient method of file-copying under Linux; in particular it is
    filesystem-aware, and can massively speed-up copies on network mounts by
    performing the copy operations server-side. However, unlike copy_file_range
    sparse files are detected and handled appropriately.
  • Support for modern filesystem features such as reflinks.
  • Optimised for ‘modern’ systems (i.e. multiple cores, copious RAM, and
    solid-state disks, especially ones connected into the main system bus,
    e.g. NVMe).
  • Optional aggressive parallelism for systems with parallel IO. Quick
    experiments on a modern laptop suggest there may be benefits to parallel
    copies on NVMe disks. This is obviously highly system-dependent.
  • Switchable ‘drivers’ to facilitate experimenting with alternative strategies
    for copy optimisation. Currently 2 drivers are available:
    • ‘parfile’: the previous hard-coded xcp copy method, which parallelises
      tree-walking and per-file copying. This is the default.
    • ‘parblock’: An experimental driver that parallelises copying at the block
      level. This has the potential for performance improvements in some
      architectures, but increases complexity. Testing is welcome.
  • Non-Linux Unix-like OSs (OS X, *BSD) are supported via fall-back operation
    (although sparse-files are not yet supported in this case).
  • Optionally understands .gitignore files to limit the copied directories.
  • Optional native file-globbing.

(Possible) future features

  • Conversion of files to sparse where appropriate, as with cp‘s
    --sparse=always flag.
  • Aggressive sparseness detection with lseek.
  • On non-Linux OSs sparse-files are not currenty supported but could be added if
    supported by the OS.

Differences with cp

  • Permissions, xattrs and ACLs are copied by default; this can be disabled with
    --no-perms.
  • Virtual file copies are not supported; for example /proc and /sys files.
  • Character files such as sockets and
    pipes are copied as
    devices (i.e. via mknod)
    rather than copying their contents as a stream.
  • The --reflink=never option may silently perform a reflink operation
    regardless. This is due to the use of
    copy_file_range
    which has no such override and may perform its own optimisations.
  • cp ‘simple’ backups are not supported, only numbered.
  • Some cp options are not available but may be added in the future.

Performance

Benchmarks are mostly meaningless, but the following are results from a laptop
with an NVMe disk and in single-user mode. The target copy directory is a git
checkout of the Firefox codebase, having been recently gc’d (i.e. a single 4.1GB
pack file). fstrim -va and echo 3 | sudo tee /proc/sys/vm/drop_caches are
run before each test run to minimise SSD allocation performance interference.

Note: xcp is optimised for ‘modern’ systems with lots of RAM and solid-state
disks. In particular it is likely to perform worse on spinning disks unless they
are in highly parallel arrays.

Local copy

  • Single 4.1GB file copy, with the kernel cache dropped each run:
    • cp: ~6.2s
    • xcp: ~4.2s
  • Single 4.1GB file copy, warmed cache (3 runs each):
    • cp: ~1.85s
    • xcp: ~1.7s
  • Directory copy, kernel cache dropped each run:
    • cp: ~48s
    • xcp: ~56s
  • Directory copy, warmed cache (3 runs each):
    • cp: ~6.9s
    • xcp: ~7.4s

NFS copy

xcp uses copy_file_range, which is filesystem aware. On NFSv4 this will result
in the copy occurring server-side rather than transferring across the network. For
large files this can be a significant win:

  • Single 4.1GB file on NFSv4 mount
    • cp: 6m18s
    • xcp: 0m37s