> On Apr 18, 2017, at 2:20 AM, Deon Brewis <[email protected]> wrote:
>
> It's not like it was subtle - it's a dead on repro. I was able to repro this
> by doing a power cycle 2 hours after shutting the app down. OSX didn't seem
> to have any interest in flushing mmap files until you soft reboot the machine.
OK, hang on — I just reread the docs on memory-mapped I/O in SQLite.
Memory-mapping is *only used for reads*, never for writes:
>> When updating the database file, SQLite always makes a copy of the page
>> content into heap memory before modifying the page. This is necessary for
>> two reasons. First, changes to the database are not supposed to be visible
>> to other processes until after the transaction commits and so the changes
>> must occur in private memory. Second, SQLite uses a read-only memory map to
>> prevent stray pointers in the application from overwriting and corrupting
>> the database file.
— https://www.sqlite.org/mmap.html
Therefore I can’t imagine how using it could trigger database corruption. It
doesn’t affect the way data is written at all!
I accept that both of you have experimentally seen that memory-mapping leads to
corruption, so I can only assume that either the above documentation is wrong,
or that there’s some subtle bug in SQLite that alters the way data is written
when memory-mapping is enabled.
—Jens
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