How can I set maxChunkSize for MMapDirectoryFactory in solr? - indexing

Question

I was reading through the MMapDirectoryFactory and how to set it as DirectoryFactory to use for indexes in my core's solrconfig.xml.
<directoryFactory name="DirectoryFactory"
class="solr.MMapDirectoryFactory">
<bool name="preload">true</bool>
</directoryFactory>
I am not able to understand nor able to find any example about how to set the maxChunkSize.
references:
https://solr.apache.org/guide/8_5/datadir-and-directoryfactory-in-solrconfig.html-
https://solr.apache.org/docs/8_5_0/solr-core/org/apache/solr/core/MMapDirectoryFactory.html

Answer 1

<directoryFactory name="DirectoryFactory"
class="${solr.directoryFactory:solr.MMapDirectoryFactory}">
<int name="maxChunkSize">1073741824</int>
<bool name="preload">true</bool>
</directoryFactory>
I hope you are looking for this.
Please note there is a validation in place for chunk size.
public MMapDirectory(Path path, LockFactory lockFactory, int maxChunkSize) throws IOException {
...
this.chunkSizePower = 31 - Integer.numberOfLeadingZeros(maxChunkSize);
assert this.chunkSizePower >= 0 && this.chunkSizePower <= 30;
}
However, that said, you ideally would not be required to modify this param for modern production systems (as they would usually be 64-bit already) per the java doc.
Create a new MMapDirectory for the named location, specifying the maximum chunk size used for memory mapping. The directory is created at the named location if it does not yet exist.
Especially on 32 bit platform, the address space can be very fragmented, so large index files cannot be mapped. Using a lower chunk size makes the directory implementation a little bit slower (as the correct chunk may be resolved on lots of seeks) but the chance is higher that mmap does not fail. On 64 bit Java platforms, this parameter should always be 1 << 30, as the address space is big enough.