Re: per table random-page-cost?

Le samedi 24 octobre 2009 01:04:19, Josh Berkus a écrit :
> Cedric,
>
> > ase is a table containing 29 GB of bytea in a database of 52 GB. Every
> > row on the 29GB table is grab only few times. And it will just renew OS
> > cache memory every time (the server have only 8GB of ram).
> > So when I remove this table (not the index) from the OS cache memory, I
> > keep more interesting blocks in the OS cache memory.
>
> effective_cache_size doesn't control what gets cached, it just tells the
> planner about it.
>
> Now, if we had an OS which could be convinced to handle caching
> differently for different physical devices, then I could see wanting
> this setting to be per-tablespace. For example, it would make a lot of
> sense not to FS-cache any data which is on a ramdisk or superfast SSD
> array. The same with archive data which you expected to be slow and
> infrequently accessed on a NAS device. If your OS can do that, while
> caching data from other sources, then it would make sense.
>
> However, I don't know any current OS which allows for this. Does anyone
> else?

Isn't it what "fadvise -dontneed" let you do ?

Josh, I talk about effective_cache_size per tablespace *exactly* for the
reason you explain.

--
Cédric Villemain
Administrateur de Base de Données
Cel: +33 (0)6 74 15 56 53
http://dalibo.com - http://dalibo.org