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Re: Tablespace-level Block Size Definitions

From: Alvaro Herrera <alvherre(at)surnet(dot)cl>
To: "Jonah H(dot) Harris" <jharris(at)tvi(dot)edu>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Tablespace-level Block Size Definitions
Date: 2005-05-31 21:10:34
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Lists: pgsql-hackers
On Tue, May 31, 2005 at 02:55:29PM -0600, Jonah H. Harris wrote:
> Hey everyone,
> I'm sure this has been thought of but was wondering whether anyone had 
> discussed the allowance of run-time block size specifications at the 
> tablespace level?  I know that a change such as this would substantially 
> impact buffer operations, transactions, access methods, the storage 
> manager, and a lot of other stuff, however it would give an 
> administrator the ability to inhance performance for specific applications.

The problem I see with this proposal is that the buffer manager knows
how to handle only a equally-sized pages.  And the shared memory stuff
gets sized according to size * num_pages.  So what happens if a certain
tablespace A with pagesize=X gets to have a lot of its pages cached,
evicting pages from tablespace B with pagesize=Y, where Y < X?

While I think it could be workable to make the buffer manager handle
variable-sized pages, it could prove difficult to handle the shared
memory.  (We can't resize it while the server is running.)

Alvaro Herrera (<alvherre[a]>)
"La principal característica humana es la tontería"
(Augusto Monterroso)

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