Re: Relation extension scalability

On 23/03/16 20:01, Robert Haas wrote:
> On Wed, Mar 23, 2016 at 2:52 PM, Petr Jelinek <petr@2ndquadrant.com> wrote:
>>> Second, if the other backend extended the relation in
>>> some other manner and did not extend the FSM, how does calling
>>> RecordAndGetPageWithFreeSpace help?  As far as I can see,
>>> GetPageWithFreeSpace and RecordAndGetPageWithFreeSpace are both just
>>> searching the FSM, so if one is stymied the other will be too.  What
>>> am I missing?
>>>
>>
>> The RecordAndGetPageWithFreeSpace will extend FSM as it calls
>> fsm_set_and_search which in turn calls fsm_readbuf with extend = true.
>
> So how does that help?  If I'm reading this right, the new block will
> be all zeroes which means no space available on any of those pages.
>

I am bit confused as to what exactly you are saying, but what will 
happen is we get back to the while cycle and try again so eventually we 
should find either block with enough free space or add new one (not sure 
if this would actually ever happen in practice in heavily concurrent 
workload where the FSM would not be correctly extended during relation 
extension though, we might just loop here forever).

--   Petr Jelinek                  http://www.2ndQuadrant.com/  PostgreSQL Development, 24x7 Support, Training &
Services