## ProFTPD module `mod_statcache`

The `mod_statcache` module is designed to cache the results of `stat(2)` and `lstat(2)` calls in shared memory, so that the results can be shared among multiple session processes. On a busy server, especially one handling many directory listings, these `stat(2)`/`lstat(2)` calls can add quite a bit of overhead.

This module is contained in the `mod_statcache.c` file for ProFTPD 1.3.x, and is not compiled by default. Installation instructions are discussed here. More examples of `mod_statcache` usage can be found here.

The most current version of `mod_statcache` is distributed with the ProFTPD source code.

## Author

Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.

## Directives

- StatCacheCapacity
- StatCacheControlsACLs
- StatCacheEngine
- StatCacheMaxAge
- StatCacheTable

## Control Actions

- `statcache`

### StatCacheCapacity

Syntax: StatCacheCapacity count
Default: StatCacheCapacity 5000
Context: server config
Module: mod_statcache
Compatibility: 1.3.4rc2 and later

The `StatCacheCapacity` directive configures the capacity of the cache, i.e. the maximum number of cache entries. By default, `mod_statcache` allocates space for 5000 cache entries.

The count value be 10 or greater. The configured count is handled as a hint; the actual allocated capacity may be rounded up to the nearest multiple of the internal block sizes.

### StatCacheControlsACLs

Syntax: StatCacheControlsACLs actions|"all" "allow"|"deny" "user"|"group" list
Default: None
Context: server config
Module: mod_statcache
Compatibility: 1.3.4rc2 and later

The `StatCacheControlsACLs` directive configures access lists of users or groups who are allowed (or denied) the ability to use the actions implemented by `mod_statcache`. The default behavior is to deny everyone unless an ACL allowing access has been explicitly configured.

If "allow" is used, then list, a comma-delimited list of users or groups, can use the given actions; all others are denied. If "deny" is used, then the list of users or groups cannot use actions all others are allowed. Multiple `StatCacheControlsACLs` directives may be used to configure ACLs for different control actions, and for both users and groups.

The action provided by `mod_statcache` is "statcache".

Examples:

      # Allow only user root to examine cache stats
      StatCacheControlsACLs all allow user root

### StatCacheEngine

Syntax: StatCacheEngine on|off
Default: StatCacheEngine off
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_statcache
Compatibility: 1.3.4rc2 and later

The `StatCacheEngine` directive enables or disables the module's caching of `stat(2)` and `lstat(2)` calls.

### StatCacheMaxAge

Syntax: StatCacheMaxAge positive-cache-age [negative-cache-age]
Default: StatCacheMaxAge 5
Context: server config
Module: mod_statcache
Compatibility: 1.3.4rc2 and later

The `StatCacheMaxAge` directive configures how long the `mod_statcache` module will keep the `stat(2)` or `lstat(2)` results cached for a given path. By default, `mod_statcache` will cache results for 5 seconds (and will cache failed `stat(2)`/`lstat(2)` calls for 1 second).

If you configure a single age parameter, then that value will be used for both positive and negative cache entries. Thus:

      StatCacheMaxAge 300

will cache successful and failed `stat(2)`/`lstat(2)` calls for 5 minutes (300 seconds).

To disable caching of failed `stat(2)`/`lstat(2)` calls entirely, use a negative-cache-age value of zero, e.g.:

      # Cache lookups for 60 seconds, and do not cache failed lookups
      StatCacheMaxAge 60 0

### StatCacheTable

Syntax: StatCacheTable path
Default: None
Context: server config
Module: mod_statcache
Compatibility: 1.3.4rc2 and later

The `StatCacheTable` directive configures a path to a file that `mod_statcache` uses for handling its cache data. The given path must be an absolute path. Note: this directive is required for `mod_statcache` to function. It is recommended that this file not be on an NFS mounted partition.

Note that statcache data is not kept across daemon stop/starts. That is, once `proftpd` is shutdown, all current statcache data is lost.

## Control Actions

### `statcache`

Syntax: ftpdctl statcache info|dump
Purpose: Display statcache information

The `statcache` action is used to display cache statistics about the statcache data maintained by `mod_statcache`. For example:

      # ftpdctl statcache info
      ftpdctl:  hits 773, misses 67: 92.0% hit rate
      ftpdctl:    expires 22, rejects 0
      ftpdctl:  current count: 1 (of 5000) (0.0% usage)
      ftpdctl:  highest count: 45 (of 5000) (0.9% usage)

To dump out the entire cache contents (not recommended on a busy server), you can use:

      # ftpdctl statcache dump

## Installation

The `mod_statcache` module is distributed with ProFTPD. For including `mod_statcache` as a statically linked module, use:

      $ ./configure --with-modules=mod_statcache

To build `mod_statcache` as a DSO module:

      $ ./configure --enable-dso --with-shared=mod_statcache

Then follow the usual steps:

      $ make
      $ make install

For those with an existing ProFTPD installation, you can use the `prxs` tool to add `mod_statcache`, as a DSO module, to your existing server:

      $ prxs -c -i -d mod_statcache.c

Note: Use of the `mod_statcache` module will interfere with the `mod_vroot` and `mod_quotatab` modules.

## Usage

The `mod_statcache` module works by allocating a SysV shared memory segment. The different `proftpd` session processes then attach to that shared memory segment so that they can share statcache results.

Example configuration:

      <IfModule mod_statcache.c>
        StatCacheEngine on
        StatCacheTable /var/run/proftpd/statcache.tab
      </IfModule>

© Copyright 2013-2017 TJ Saunders
All Rights Reserved