Arvados

Background:

arv-mount has a block cache, which improves performance when the same blocks are read multiple times. However:

• Currently a new arv-mount process is started for each Crunch task execution. This means tasks don't share a cache, even if they're running at the same time.
• In the common case where multiple crunch tasks run at the same time and use the same data, we have multiple arv-mount processes each retrieving and caching its own copy of the same data blocks.

Proposed improvement:

• Use large swap on worker nodes (preferably SSD). (We already do this for other reasons.)
• Set up a large tmpfs on worker nodes and use it as crunch job scratch space. (This already gets cleared at the beginning of a job to avoid leakage between jobs/users.)
• Use a directory in that tmpfs as an arv-mount cache. This makes it feasible to use a large cache size, and makes it easy to share the cache between multiple arv-mount processes.

Implementation notes:

• Rely on unix permissions for cache privacy. (Warn if the cache dir's mode & 0007 != 0, but go ahead anyway: there will be cases where that would be useful and not dangerous.)
• Use flock() to avoid races and duplicated effort. (If arv-mount 1 is writing a block to the cache, then arv-mount 2 should wait for arv-mount 1 to finish then read from the cache, rather than fetch its own copy.)
• Do not clean up cache dir at start/exit, at least by default (the general idea is to share with past/future arv-mount procs). An optional --cache-clear-atexit flag would be nice to have.
• Measuring/limiting cache size could be interesting
• Delete & replace upon finding a corrupt/truncated cache entry

Integration:

• The default Keep mount on shell nodes should use a filesystem cache, assuming there is an appropriate filesystem for it (i.e., something faster than network: tmpfs, SSD, or at least a disk with async/barriers=0).
• crunch-job should create a per-job temp dir on each node during the "install" phase, and point all arv-mount processes to it.