fastrm - Online in the Cloud

PROGRAM:

NAME

fastrm - Quickly remove a list of files

SYNOPSIS

fastrm [-de] [-c|-cI] [-s|-sM] [-u|-uN] base-directory

DESCRIPTION

fastrm reads a list of either file names or storage API tokens, one per line, from its
standard input and removes them. Storage API tokens are removed via the SMcancel()
interface. fastrm does not delete files safely or with an eye to security, but rather
cuts every corner it can to delete files as fast as it can. It should therefore never be
run on publically writable directories, or in any other environment where a hostile party
may control the directory structure in which it is working.

If a file name is not an absolute path name, it is considered to be relative to base-
directory as given on the command line. The base-directory parameter must be a simple
absolute pathname (it must not contain multiple consecutive slashes or references to the
special directories "." or "..").

fastrm is designed to be faster than the typical "| xargs rm" pipeline when given a sorted
list of file names as input. For example, fastrm will usually chdir(2) into a directory
before removing files from it, meaning that if its input is sorted, most names passed to
unlink(2) will be simple names. This can substantially reduce the operating system
overhead from directory lookups.

fastrm assumes that its input is valid and that it is safe to call unlink(2) on every file
name it is given. As a safety measure, however, fastrm when running as root will check
with stat(2) that a file name doesn't specify a directory before removing it. (In some
operating systems, root is allowed to unlink directories, even directories which aren't
empty, which can cause file system corruption.)

The input to fastrm should always be sorted -- or even better be in the order file names
are output by find(1) -- if speed is an issue and the input isn't solely storage API
tokens. (It deals fine with unsorted input, but is unlikely to be any faster in that case
than a simple "| xargs rm" command.) Sorting may even slightly speed up the removal of
storage API tokens due to caching effects, since sorting will tend to keep all of the
tokens from a particular storage method together.

Various additional optimizations for removing files can be turned on and/or tuned with
options (see below). Which options will be most effective depends heavily on the
underlying structure of the file system, the way in which directories are stored and
searched, and similar, often underdocumented, operating system implementation details.
The more sophisticated the underlying operating system and file system, the more likely
that it will already perform the equivalent of these optimizations internally.

OPTIONS

-c[I]
Controls when fastrm calls chdir(2). If the number of files to be unlinked from a
given directory is at least I, then fastrm will change to that directory before
unlinking those files. Otherwise, it will use either the absolute path names or a
path name relative to the current directory (whichever is likely more efficient). The
I parameter is optional; if just -c is given, -c1 is assumed, which will cause fastrm
to always chdir before calling unlink(2). The default is -c3. Use -c0 to prevent
fastrm from ever using chdir(2).

-d Don't remove any files. Instead, print a list of the files that would be removed to
standard output. Each line contains either the current directory of fastrm at the
time it would do the unlink and the relative path name it would pass to unlink(2) as
two fields separated by whitespace and a "/", the absolute path name (as a single
field) that would be passed to unlink(2), or the string "Token" and the storage API
token that would be removed.

-e Treat an empty input file as an error. This is most useful when fastrm is last in a
pipeline after a preceding sort(1) command, ensuring that fastrm will fail if the sort
fails.

-s[M]
When -s is given and the number of files to remove in a directory is greater than M,
rather than remove files in the order given, fastrm will open the directory and read
it, unlinking files in the order that they appear in the directory. On systems with a
per-process directory cache or that use a linear search to find files in a directory,
this should make directory lookups faster. The M parameter is optional; if just -s is
given, -s5 is assumed.

When this option is in effect, fastrm won't attempt to remove files that it doesn't
see in the directory, possibly significantly speeding it up if most of the files to be
removed have already been deleted. However, using this option requires fastrm to do
more internal work and it also assumes that the order of directory listings is stable
in the presence of calls to unlink(2) between calls to readdir(3). This may be a
dangerous assumption with some sophisticated file systems (and in general this option
is only useful with file systems that use unindexed linear searches to find files in
directories or when most of the files to be removed have already been deleted).

This optimization is off by default.

-u[N]
Specifying this option promises that there are no symbolic links in the directory tree
from which files are being removed. This allows fastrm to make an additional
optimization to its calls to chdir(2), constructing a relative path using "../.." and
the like to pass to chdir(2) rather than always using absolute paths. Since this
reduces the number of directory lookups needed with deeply nested directory structures
(such as that typically created by traditional news spool storage), it can be a
significant optimization, but it breaks horribly in the presence of symbolic links to
directories.

When -u is given, fastrm will use at most N levels of ".." segments to construct
paths. N is optional; if just -u is given, -u1 is assumed.

This optimization is off by default.

fastrm also accepts -a and -r options, which do nothing at all except allow you to say
"fastrm -usa", "fastrm -usr", or "fastrm -user". These happen to often be convenient sets
of options to use.

EXIT STATUS

fastrm exits with a status of zero if there were no problems, and an exit status of 1 if
something went wrong. Attempting to remove a file that does not exist is not considered a
problem.

EXAMPLES

fastrm is typically invoked by INN via expirerm(8) using a command like:

fastrm -e <patharticles in inn.conf> < expire.list

To enable all optimizations and see the affect on the order of removal caused by -s, use:

fastrm -d -s -e -u <patharticles> < expire.list

If your file system has indexed directory lookups, but you have a deeply nested directory
structure, you may want to use a set of flags like:

fastrm -e -u3 <patharticles> < expire.list

to strongly prefer relative paths but not to use readdir(2) to order the calls to
unlink(2).

You may want to edit expirerm(8) to change the flags passed to fastrm.

WARNINGS

fastrm cuts corners and does not worry about security, so it does not use chdir(2) safely
and could be tricked into removing files other than those that were intended if run on a
specially constructed file tree or a file tree that is being modified while it is running.
It should therefore never be used with world-writable directories or any other directory
that might be controlled or modified by an attacker.

NOTES

fastrm defers opening the storage subsystem or attempting to parse any INN configuration
files until it encounters a token in the list of files to remove. It's therefore possible
to use fastrm outside of INN as a general fast file removal program.

HISTORY

fastrm was originally written by <[email protected]>. This manual page was rewritten in
POD by Russ Allbery <[email protected]> for InterNetNews.

$Id: fastrm.pod 9767 2014-12-07 21:13:43Z iulius $

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