Plugin: mmapstorage

• infos = Information about the mmapstorage plugin is in keys below
• infos/author = Mihael Pranjić mpran.nosp@m.j@li.nosp@m.mun.o.nosp@m.rg
• infos/licence = BSD
• infos/needs =
• infos/provides = storage/mmapstorage
• infos/recommends =
• infos/placements = getstorage setstorage
• infos/status = experimental
• infos/metadata =
• infos/description = high performance storage using memory mapped files

Introduction

This is a high performance storage plugin that supports full Elektra semantics.

Format

The storage format uses Elektra's in-memory data layout and employs the mmap() system call to read/write data. The format is not portable across different architectures/platforms. The format can be seen as a memory dump of a keyset. Therefore, the files must not be edited by hand. Files written by mmapstorage are not intended to be human-readable.

Usage

Mount mmapstorage using kdb mount:

sudo kdb mount config.mmap user:/tests/mmapstorage mmapstorage

Unmount mmapstorage using kdb umount:

sudo kdb umount user:/tests/mmapstorage

Compiling

The mmapstorage has two compilation variants:

1. mmapstorage
2. mmapstorage_crc

The mmapstorage will always be compiled on a supported system (see Dependencies). When zlib is available, we will additionally compile the mmapstorage_crc variant. The first variant does not do a CRC32 checksum of the critical data, while the second variant always checks the CRC32 checksum for additional security.

Installation

See installation. The mmapstorage variant is part of the libelektra5 package and the mmapstorage_crc is part of the libelektra5-extra package.

Dependencies

POSIX compliant system (including XSI extensions).

Additionally, zlib is needed for the mmapstorage_crc compilation variant: zlib1g-dev or zlib-devel.

Examples

# Mount mmapstorage to `user:/tests/mmapstorage`
sudo kdb mount config.mmap user:/tests/mmapstorage mmapstorage
 
# Add some values via `kdb set`
kdb set user:/tests/mmapstorage 'Some root key'
kdb set user:/tests/mmapstorage/dir 'Directory within the hierarchy.'
kdb set user:/tests/mmapstorage/dir/leaf 'A leaf node holding some valuable data.'
kdb meta-set  user:/tests/mmapstorage/dir/leaf superMetaKey 'Metadata is supported too.'
 
# List the configuration tree below `user:/tests/mmapstorage`
kdb ls user:/tests/mmapstorage
#> user:/tests/mmapstorage
#> user:/tests/mmapstorage/dir
#> user:/tests/mmapstorage/dir/leaf
 
# Retrieve the new values
kdb get user:/tests/mmapstorage
#> Some root key
kdb get user:/tests/mmapstorage/dir
#> Directory within the hierarchy.
kdb get user:/tests/mmapstorage/dir/leaf
#> A leaf node holding some valuable data.
kdb meta-get  user:/tests/mmapstorage/dir/leaf superMetaKey
#> Metadata is supported too.
 
# Undo modifications to the database
kdb rm -r user:/tests/mmapstorage
 
# Unmount mmapstorage
sudo kdb umount user:/tests/mmapstorage

Limitations

Mapped files shall not be altered, otherwise the behavior is undefined.

The mmap() system call only supports regular files and so does the mmapstorage plugin with one notable exception: The plugin detects when it is called with the files /dev/stdin and /dev/stdout and makes an internal copy. This makes the plugin compatible with kdb import and kdb export.