- 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 = unittest 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:
- mmapstorage
- 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
.