Re: Free Space Map thoughts

Simon Riggs wrote:
> On Thu, 2007-11-08 at 21:50 -0300, Alvaro Herrera wrote:
>> Simon Riggs wrote:
>>
>>> Presumably we would not store an FSM for small tables? On the basis that
>>> the purpose of the FSM is to save on pointless I/O there must be a size
>>> of table below which an FSM is just overhead.
>> Hmm, do you mean that we would open and verify every page of a small
>> relation until we find one with free space?  That doesn't sound very
>> good.
> 
> I was trying to guess at Heikki's reason for saying the FSM should be in
> a separate file. If we have one extra file per table that seems like a
> huge number of additional files, with space and fsync implications.

You need the same amount of space either way. Well, I guess additional 
files consume some space in the filesystem directory structures. I doubt 
that matters.

You do need more fsyncs, and more overhead when creating/dropping 
relations, to create/unlink the FSM files. I don't know how significant 
that is.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com