[ikawe]

SQLite has different “modes” to facilitate recovery, this new WAL2 mode addresses the problem in WAL (version 1) where the recovery log could potentially grow very large.

It’s solving a real problem, but not considered stable for production yet.

[bob1029]

I am curious what the situation is where the recovery log grows to large size and what the actual consequence of this would be.

We have been using SQLite in WAL mode for over half a decade and never witnessed this. Several of our databases can see concurrent access from hundreds of users with transactions in the 1-10 megabyte range, so I find it a bit odd this never came up.

[zimpenfish]

> I am curious what the situation is where the recovery log grows to large size

From the link, "if a writer writes to the database while a checkpoint is ongoing [...] it does mean that the wal file may grow indefinitely if the checkpointer never gets a chance to finish without a writer appending to the wal file. There are also circumstances in which long-running readers may prevent a checkpointer from checkpointing the entire wal file - also causing the wal file to grow indefinitely in a busy system."

[Seattle3503]

> what the actual consequence of this would be.

SQLite runs in a surprising number of places. In an embedded environment disk space may be limited.

[pkhuong]

I've seen the WAL grow when there's a constant write load and a long background read transaction keeps an old version alive. If you also never reach a point where there are 0 connections to the DB, you can keep this large WAL file around (most of it is unused) for a long time.

I've had to restart services when WAL files had grown to multiple GBs and wouldn't shrink.

[muttled]

I ran into this when I was importing about 1TB into a DB while simultaneously reading from it and performing tasks. This was all in a dev environment, but it did come up.

[jessermeyer]

Citation needed.

[zimpenfish]

For which claim?

The recovery log growing large is explained in [1] - "it does mean that the wal file may grow indefinitely [...] There are also circumstances [...] causing the wal file to grow indefinitely in a busy system."

Not ready for production is [2] - although that is from 2 years ago, the WAL2 branch still isn't merged to trunk [3] which you'd expect if it was ready to go, I think?

[1] https://www.sqlite.org/cgi/src/doc/wal2/doc/wal2.md [2] https://sqlite.org/forum/forumpost/17249fb83a?t=c&unf [3] https://www.sqlite.org/cgi/src/timeline?r=wal2