On Sat, Mar 26, 2011 at 8:05 PM, Jan Wieck <JanWieck@yahoo.com> wrote:
> On 3/26/2011 12:12 PM, Simon Riggs wrote:
>>
>> On Sat, Mar 26, 2011 at 2:30 PM, Jan Wieck<JanWieck@yahoo.com> wrote:
>>
>>> My current idea for a fix is to modify lazy_truncate_heap(). It does
>>> acquire
>>> and release the exclusive lock, so it should be possible to do this in
>>> smaller chunks, releasing and reacquiring the lock so that client
>>> transactions can get their work done as well.
>>
>> Agreed, presumably with vacuum delay in there as well?
>
> Not sure about that. My theory is that unless somebody needs access to that
> table, just have at it like it is now.
>
> The current implementation seems to assume that the blocks, checked for
> being empty, are still found in memory (vacuum just scanned them). And that
> seems to be correct most of the time, in which case adding vacuum delay only
> gives more time that the blocks get evicted and have to be read back in.
I think someone fairly clever already thought of that.
vacuum_cost_page_hit = 1 by default, so the cost of accessing pages
still in memory is 1/10th the cost of disk access. So we will only
perform the delay for each chunk if we had to read it from disk.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
