The algo is today, after a failure detection:
- split the logs
- when all the logs are split, assign the regions
But some regions can have no entries at all in the HLog. There are many reasons for this:
- kind of reference or historical tables. Bulk written sometimes then read only.
- sequential rowkeys. In this case, most of the regions will be read only. But they can be in a regionserver with a lot of writes.
- tables flushed often for safety reasons. I'm thinking about meta here.
For meta; we can imagine flushing very often. Hence, the recovery for meta, in many cases, will be the failure detection time.
There are different possible algos:
A new task is added, in parallel of the split. This task reads all the HLog. If there is no entry for a region, this region is assigned.
Cons: We will need to read all the files. Add a read.
The master writes in ZK the number of log files, per region.
When the regionserver starts the split, it reads the full block (64M) and decrease the log file counter of the region. If it reaches 0, the assign start. At the end of its split, the region server decreases the counter as well. This allow to start the assign even if not all the HLog are finished. It would allow to make some regions available even if we have an issue in one of the log file.
Cons: add something to do for the region server. Requites to read the whole file before starting to write.
Add some metadata at the end of the log file. The last log file won't have meta data, as if we are recovering, it's because the server crashed. But the others will. And last log file should be smaller (half a block on average).
Option 4) Still some metadata, but in a different file. Cons: write are increased (but not that much, we just need to write the region once). Pros: if we lose the HLog files (major failure, no replica available) we can still continue with the regions that were not written at this stage.
I think it should be done, even if none of the algorithm above is totally convincing yet. It's linked as well to locality and short circuit reads: with these two points reading the file twice become much less of an issue for example. My current preference would be to open the file twice in the region server, once for splitting as of today, once for a quick read looking for unused regions. Who knows, may be it would even be faster this way, the quick read thread would warm-up the different caches for the splitting thread.