50 releases (21 breaking)
0.22.0 | Oct 28, 2024 |
---|---|
0.21.2 | Jun 29, 2024 |
0.20.4 | Feb 23, 2024 |
0.16.0 | Dec 20, 2023 |
0.1.1-alpha.3 | Nov 21, 2018 |
#42 in Filesystem
718 downloads per month
140KB
2.5K
SLoC
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).
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:
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:
yay -S xcp
NetBSD
xcp
is available on NetBSD from the official repositories. To install it, simply run:
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, unlikecopy_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
: ~6.2sxcp
: ~4.2s
- Single 4.1GB file copy, warmed cache (3 runs each):
cp
: ~1.85sxcp
: ~1.7s
- Directory copy, kernel cache dropped each run:
cp
: ~48sxcp
: ~56s
- Directory copy, warmed cache (3 runs each):
cp
: ~6.9sxcp
: ~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
: 6m18sxcp
: 0m37s
Dependencies
~21–30MB
~555K SLoC