The Database Engine works like a traditional database. There is some amount of RAM dedicated to data caching and indexing and the rest of the data reside compressed on disk. The number of history entries is not fixed in this case, but depends on the configured disk space and the effective compression ratio of the data stored.
With the DB engine memory mode the metric data are stored in database files. These files are organized in pairs, the datafiles and their corresponding journalfiles, e.g.:
datafile-1-0000000001.ndf journalfile-1-0000000001.njf datafile-1-0000000002.ndf journalfile-1-0000000002.njf datafile-1-0000000003.ndf journalfile-1-0000000003.njf ...
They are located under their host’s cache directory in the directory
(e.g. for localhost the default location is
/var/cache/netdata/dbengine/*). The higher
numbered filenames contain more recent metric data. The user can safely delete some pairs
of files when netdata is stopped to manually free up some space.
Users should back up their
./dbengine folders if they consider this data to be important.
There is one DB engine instance per netdata host/node. That is, there is one
per node, and all charts of
dbengine memory mode in such a host share the same storage space
and DB engine instance memory state. You can select the memory mode for localhost by editing
netdata.conf and setting:
[global] memory mode = dbengine
For setting the memory mode for the rest of the nodes you should look at streaming.
history configuration option is meaningless for
memory mode = dbengine and is ignored
for any metrics being stored in the DB engine.
All DB engine instances, for localhost and all other streaming recipient nodes inherit their
[global] page cache size = 32 dbengine disk space = 256
The above values are the default and minimum values for Page Cache size and DB engine disk space quota. Both numbers are in MiB. All DB engine instances will allocate the configured resources separately.
page cache size option determines the amount of RAM in MiB that is dedicated to caching
netdata metric values themselves.
dbengine disk space option determines the amount of disk space in MiB that is dedicated
to storing netdata metric values and all related metadata describing them.
The DB engine stores chart metric values in 4096-byte pages in memory. Each chart dimension gets its own page to store consecutive values generated from the data collectors. Those pages comprise the Page Cache.
When those pages fill up they are slowly compressed and flushed to disk.
It can take
4096 / 4 = 1024 seconds = 17 minutes, for a chart dimension that is being collected
every 1 second, to fill a page. Pages can be cut short when we stop netdata or the DB engine
instance so as to not lose the data. When we query the DB engine for data we trigger disk read
I/O requests that fill the Page Cache with the requested pages and potentially evict cold
(not recently used) pages.
When the disk quota is exceeded the oldest values are removed from the DB engine at real time, by automatically deleting the oldest datafile and journalfile pair. Any corresponding pages residing in the Page Cache will also be invalidated and removed. The DB engine logic will try to maintain between 10 and 20 file pairs at any point in time.
The Database Engine uses direct I/O to avoid polluting the OS filesystem caches and does not generate excessive I/O traffic so as to create the minimum possible interference with other applications.
Using memory mode
dbengine we can overcome most memory restrictions and store a dataset that
is much larger than the available memory.
There are explicit memory requirements per DB engine instance, meaning per netdata node (e.g. localhost and streaming recipient nodes):
page cache sizemust be at least
#dimensions-being-collected x 4096 x 2bytes.
#pages-on-disk x 4096 x 0.06bytes of RAM are allocated for metadata.
roughly speaking this is 6% of the uncompressed disk space taken by the DB files.
for very highly compressible data (compression ratio > 90%) this RAM overhead is comparable to the disk space footprint.
An important observation is that RAM usage depends on both the
page cache size and the
dbengine disk space options.