This blog discusses some important aspects of GCache.

Why do we need GCache?

Percona XtraDB Cluster is a multi-master topology, where a transaction executed on one node is replicated on another node(s) of the cluster. This transaction is then copied over from the group channel to Galera-Cache followed by apply action.

The cache can be discarded immediately once the transaction is applied, but retaining it can help promote a node as a DONOR node serving write-sets for a newly booted node.

So in short, GCache acts as a temporary storage for replicated transactions.

How is GCache managed?

Naturally, the first choice to cache these write-sets is to use memory allocated pool, which is governed by gcache.mem_store. However, this is deprecated and buggy and shouldn’t be used.

Next …

When a write query is sent to Percona XtraDB Cluster all the nodes store the writeset on a file called gcache. By default the name of that file is galera.cache and it is stored in the MySQL datadir. This is a very important file, and as usual with the most important variables in MySQL, the default value is not good for high-loaded servers. Let’s see why it’s important and how can we calculate a correct value for the workload of our cluster.

What’s the gcache?
When a node goes out of the cluster (crash or maintenance) it obviously stops receiving changes. When you try to reconnect the node to the cluster the data will be outdated. The joiner node needs to ask a donor to send the changes happened during the downtime.

The donor …