CouchDB opened a new way of thinking - map/reduce, document storage, a streaming changes feed, and easy replication. I was and continue to be a huge fan of couchdb and the brave new world of storage options it helped to open.
In practice there are two gotchas that are so painful I am looking for a replacement with a different featureset than couchdb provides. The location tracking project icecondor.com uses couchdb to store 20,000 new records per day. It has more write traffic than read traffic and runs on modest hardware.
Those two gotchas are:
1. View Index updates.
While I have a vague understanding of why view index updates are slow and bulky and important, in practice it is unworkable. Every write sets up a trap for the first reader to come along after the write. The more writes there are, the bigger the trap for the first reader which has to wait on the couchdb process that refreshes the view index on an as-needed basis. I believe this trade-off was made to keep writes fast. No need to update the view index until all writes are actually complete, right?
Write traffic is heavier than read traffic and the time needed for that index refresh causes the webapp to crash because its not setup to handle timeouts from a database query. The workaround is as hackish as one can imagine - cron jobs to hit every map/reduce query to keep indexes fresh.
2. Append only database file
Append only is in theory a great way to ensure on-disk reliability. A system crash during an append should only affect that append. Its a crash during an update to existing parts of the file that risks the integrity of more than whats being updated. With so many layers of caching and optimizations in the kernel and the filesystem and now in the workings of SSD drives, I'm not sure append-only gives extra protection anymore.
What it does do is a create a huge operational headache. The on-disk file can never grow beyond half the available storage space. Record deletion uses new disk space and if the half-full mark approaches, vacuuming must be done. The entire database is rewritten to the filesystem, leaving out no longer needed records. If the data file should happen to grow beyond half the partition, the system has esentially crashed because there is no way to compact the file and soon the partition will be full. This is a likely scenario when there is a lot of record deletion activity.
The system in question does a lot of writes of temporary data that is followed up by deletes a few days later. There is also a lot of permanent storage that hardly gets used. Rewriting every byte of the records that are long-lived due to compaction is an enormous amount of wasted I/O - doubly so given SSD drives have a short write-cycle lifespan
See the next post for what I'd like in a storage system.