Local database

Design document
Revision	v1
Status	proposed
Review	create new issue

All hosts in a pool use the shared database by sending queries to the pool master. This creates a performance bottleneck as the pool size increases. All hosts in a pool receive a database backup from the master periodically, every couple of hours. This creates a reliability problem as updates may be lost if the master fails during the window before the backup.

The reliability problem can be avoided by running with HA or the redo log enabled, but this is not always possible.

We propose to:

• adapt the existing event machinery to allow every host to maintain an up-to-date database replica;
• actively cache the database locally on each host and satisfy read operations from the cache. Most database operations are reads so this should reduce the number of RPCs across the network.

In a later phase we can move to a completely distributed database.

Replicating the database

We will create a database-level variant of the existing XenAPI event.from API. The new RPC will block until a database event is generated, and then the events will be returned using the existing “redo-log” event types. We will add a few second delay into the RPC to batch the updates.

We will replace the pool database download logic with an event.from-like loop which fetches all the events from the master’s database and applies them to the local copy. The first call will naturally return the full database contents.

We will turn on the existing “in memory db cache” mechanism on all hosts, not just the master. This will be where the database updates will go.

The result should be that every host will have a /var/xapi/state.db file, with writes going to the master first and then filtering down to all slaves.

Using the replica as a cache

We will re-use the Disaster Recovery multiple database mechanism to allow slaves to access their local database. We will change the defalult database “context” to snapshot the local database, perform reads locally and write-through to the master.

We will add an HTTP header to all forwarded XenAPI calls from the master which will include the current database generation count. When a forwarded XenAPI operation is received, the slave will deliberately wait until the local cache is at least as new as this, so that we always use fresh metadata for XenAPI calls (e.g. the VM.start uses the absolute latest VM memory size).

We will document the new database coherence policy, i.e. that writes on a host will not immediately be seen by reads on another host. We believe that this is only a problem when we are using the database for locking and are attempting to hand over a lock to another host. We are already using XenAPI calls forwarded to the master for some of this, but may need to do a bit more of this; in particular the storage backends may need some updating.