I think you're mixing up database size and transaction size.

WAL starts having issues when a single transaction commits 100's of megabytes 
or more of data.  I believe there is no issue with very large databases, 
provided the actual transactions are relatively small.

> -----Original Message-----
> From: [email protected] [mailto:sqlite-users-
> [email protected]] On Behalf Of Aemon Cannon
> Sent: Tuesday, February 14, 2012 3:03 PM
> To: [email protected]
> Subject: [sqlite] WAL mode, references to IO errors
> 
> Hello,
> 
> In reading about WAL mode, I found the following passage disconcerting:
> 
> "WAL works best with smaller transactions. WAL does not work well for
> very large transactions. For transactions larger than about 100
> megabytes, traditional rollback journal modes will likely be faster.
> For transactions in excess of a gigabyte, WAL mode may fail with an I/O
> or disk-full error.
> It is recommended that one of the rollback journal modes be used for
> transactions larger than a few dozen megabytes."
> - sqlite.org/wal.html
> 
> 
> On the other hand, I've read anecdotal reports from users on this list
> who are using WAL mode with large databases (10s of gigabytes).
> Is wal.html a bit out of date?
> What exactly is meant by "may fail with an I/O or disk-full error"? Is
> this just saying that if your WAL file grows larger than your available
> disk space, you're out of luck?
> 
> I'm very interested in the concurrency benefits of WAL mode, but I want
> to be sure it's a safe choice.
> 
> Thanks!
> Aemon
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