Do not forget to set -XX:MaxDirectMemorySize for embedded databases

Since 2.2 version of OrientDB, we have migrated from the usage of sun.misc.Unsafe for direct memory management to the usage of direct ByteBuffer-s. But there is a small side effect which affects all users of embedded databases.  The amount of direct memory which can be allocated in total, inside of all ByteBuffer-s, limited either to the maximum allowed heap size or to size specified in -XX:MaxDirectMemorySize parameter.

What would it be if you had not set this parameter for embedded database ? Well, nothing dramatical, OrientDB will detect the absence of this parameter and decrease the size of disk cache. As the result, all database operation would be much slower and overall system performance would suffer.

If there are no any specific requirements for your application we recommend to set this parameter to 512g for 64-bit JVMs and to 2g for 32-bit JVMs as we do inside of server startup script.

The setting of this parameter to 512g does not mean that OrientDB will consume all 512GB memory. The size of direct memory consumed by OrientDB is limited by the size of disc cache, more details on this topic may be found there.

How to calculate maximum amount of direct memory consumed by OrientDB

Many users ask how much of memory will be consumed by OrientDB and which settings affects this number. This question becomes even more actual since 2.2 release. In the new release, we allocate memory by big chunks (about of 1Gb size each) and then split it between threads on demand.

So how to calculate the maximum amount of memory which will be consumed by OrientDB ? That is simple. OrientDB uses both memory heap and direct memory. Direct memory is used for the disk cache.

Memory consumed by disk cache may be calculated looking on the value of following configuration parameter storage.diskCache.bufferSize which shows the maximum amount of memory consumed by disk cache in megabytes.
We need to increase this value till it will not be divided to the 1GB without a reminder (size of a chunk of memory allocated by the pool at once).

The rest is easy. Add value which you calculated above and add the amount of memory consumed by memory heap and you will get the maximum amount of memory which will be consumed by OrientDB.

P.S.1 Maximum size of memory chunk in bytes which will be allocated at once by OrientDB may be set using property memory.chunk.size .

P.S.2: You may find actual amount of direct memory consumed by OrientDB accessing properties allocatedMemory

allocatedMemoryInMB allocatedMemoryInGB of com.orientechnologies.common.directmemory:type=OByteBufferPoolMXBean MBean.

How to change disk cache size at runtime in OrientDB

Since 2.1.16 and 2.2 versions of  OrientDB, it will be possible to change the size of direct memory disk cache at runtime.

That is quite simple to do, merely call com.orientechnologies.orient.core.config.OGlobalConfiguration#DISK_CACHE_SIZE.setValue() at any place in your program and size of disk cache will be changed (passed in value is cache size in megabytes). You may do this inside of your code or using OrientDB console or Studio.

There are couple of nuances of usage of given feature:

1. If you increase cache size that is pretty cheap operation.
2. If you decrease cache size it may take some time because previously loaded pages have to be freed.
3. If you decrease cache size you may get (but the probability that it may happen is really low) IllegalStateException which tells you that cache does not have enough memory to keep pinned pages inside of RAM

Pinned pages is a special area of disk cache which unloaded from RAM only when OrientDB storage is closed. It is used in OrientDB clusters and hash index to speed up a performance of storage operations.