On 03/07/2014 03:48 PM, Robert Haas wrote:
On Fri, Mar 7, 2014 at 4:34 AM, Heikki Linnakangas
<hlinnakan...@vmware.com>  wrote:
>Hmm. You suggested ensuring that a scan always has at least a pin, and split
>takes a vacuum-lock. That ought to work. There's no need for the more
>complicated maneuvers you described, ISTM that you can just replace the
>heavy-weight share lock with holding a pin on the primary page of the
>bucket, and an exclusive lock with a vacuum-lock. Note that
>_hash_expandtable already takes the exclusive lock conditionally, ie. if it
>doesn't get the lock immediately it just gives up. We could do the same with
>the cleanup lock.
We could try that.  I assume you mean do*just*  what you describe
here, without the split-in-progress or moved-by-split flags I
suggested.


Yep.

The only issue I see with that is that instead of everyone
piling up on the heavyweight lock, a wait which is interruptible,
they'd all pile up on the buffer content lwlock, a wait which isn't.
And splitting a bucket can involve an arbitrary number of I/O
operations, so that's kind of unappealing.  Even checkpoints would be
blocked until the bucket split completed, which seems unfortunate.

Hmm. I doubt that's a big deal in practice, although I agree it's a bit ugly.

Once we solve the crash-safety of splits, we actually have the option of doing the split in many small steps, even when there's no crash involved. You could for example grab the vacuum-lock, move all the tuples in the first 5 pages, and then release the lock to give other backends that are queued up a chance to do their scans/insertions. Then re-acquire the lock, and continue where you left. Or just bail out and let the next vacuum or insertion to finish it later.

- Heikki


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