The HdfsTransactionLog implementation keeps a Hadoop FSDataOutputStream open for its whole lifetime, which consumes two threads on the HDFS data node server (dataXceiver and packetresponder) even once the Solr tlog has finished being written to.
This means for a cluster with many indexes on HDFS, the number of Xceivers can keep growing and eventually hit the limit of 4096 on the data nodes. It's especially likely for indexes that have low write rates, because Solr keeps enough tlogs around to contain 100 documents (up to a limit of 10 tlogs). There's also the issue that attempting to write to a finished tlog would be a major bug, so closing it for writes helps catch that.
Our cluster during testing had 100+ collections with 100 shards each, spread across 8 boxes (each running 4 solr nodes and 1 hdfs data node) and with 3x replication for the tlog files, this meant we hit the xceiver limit fairly easily and had to use the attached patch to ensure tlogs were closed for writes once finished.
The patch introduces an extra lifecycle state for the tlog, so it can be closed for writes and free up the HDFS resources, while still being available for reading. I've tried to make it as unobtrusive as I could, but there's probably a better way. I have not changed the behaviour of the local disk tlog implementation, because it only consumes a file descriptor regardless of read or write.
nb We have decided not to use Solr-on-HDFS now, we're using local disk (for various reasons). So I don't have a HDFS cluster to do further testing on this, I'm just contributing the patch which worked for us.