Hi Pavel,
thanks for helping me out!
>> Is this intended behaviour? How does the pragma/async io module fit in,
>> then?
>>
> I believe you've got something wrong in your tests. Because with
> asynchronous module (if you indeed used its VFS when created your
> connections) no actual disk I/O happens in the same thread as insert
> command.
Yes, I thought so as well.
But when I profiled with google perftools, unixSync showed up in the
call stack below the place where the insert commands were executed. I
have started async io by calling sqlite3async_initialize before opening
the database. It returned SQLITE_OK. Just to make sure that I'm not
looking at the wrong code, I have inserted a hardcoded breakpoint
(asm("int3);) into the flush-function and the debugger stops frequently
at that point.
Compiling with -DSQLITE_NO_SYNC is running an order of magnitude faster
than all combinations of PRAGMA synchronous, PRAGMA journal_mode and
async io that I could think of.
Unfortunately all of this is embedded in a large scale application where
I can't easily extract a small example demonstrating the effect. The
code uses the Sql module from Qt with a sqlite backend compiled as a
plugin. Maybe something is lost along these lines.
I'll see if I can whip up anything smaller without Qt inbetween.
> With PRAGMA synchronous = off the only syncs I can think of
> being called is sync on directory where journal is created and
> deleted. If you don't care about database corruption and if you don't
> need to do rollbacks then maybe you need to try PRAGMA journal_mode =
> off too? Or at least PRAGMA journal_mode = persist if you still need
> to do rollbacks.
>
I don't need to do rollbacks and have tried journal_mode=off vs
journal_mode=memory. The former didn't make much of a difference, so I
opted for the latter, just in case.
See, the unixSync-function says:
static int unixSync(sqlite3_file *id, int flags){
/* ... */
int isDataOnly = (flags&SQLITE_SYNC_DATAONLY);
int isFullsync = (flags&0x0F)==SQLITE_SYNC_FULL;
/* ... */
rc = full_fsync(pFile->h, isFullsync, isDataOnly);
/* ... */
}
And full_fsync does call fsync unless compiled with -DSQLITE_NO_SYNC
I still need to check why unixSync is triggered by INSERT. I had
problems to get debugging info going and therefore had to resort to
looking at machine code (hence the hardcoded breakpoint).
Cheers,
Benjamin
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