Persistency and Performance Guarantees

MemCP gives the user several Guarantees for persistency and performance. These are the guarantees:

Persistency Guarantees

There are three persistency modes per table which are:

all data is held in memory and only in memory
in case of a crash, all data is gone
the schema is saved on disk
after a recovery, the table starts empty
fastest way to store data
use it for session data, observer handles, caches and other data that can be recreated by the software

all data is held in memory
the main storage is mirrored on disk
the delta storage is RAM-only
a main storage rebuild is triggered every 15 minutes, so data older than 15 minutes are guaranteed to be persistent
in case of a crash, the delta storage is gone, the main storage is recovered
after a recovery, some datasets or deletions that happend in the last 15 minutes before the crash may be gone
extremely fast way to store data
use it for frequently updated tables with unimportand data like usage statistics or sensor data

all data is held in memory
the main storage is mirrored on disk
changes to the delta storage are logged on disk files
an operation succeeds even if data is not synced to disk permanently yet
in case of a crash, data might be recovered
- in case of a crash of the process, all data will be recovered
- in case of a power outage or kernel crash, data might be lost
allows buffering of the log file in return for the risk of data loss
use it for data where you need a high update performance but cannot afford losing the last 15 minutes of your work

MemCP guarantees that:

Insert/Update without any unique key check or foreign key check will scale over shards
Aggregate functions like SUM, AVG will scale perfectly with the amount of CPUs

With these guarantees, you can add more CPU cores whenever you have more data.