Re: Configuring for maximum memory usage

Hi Scott,

Thaks for the clear answers!

Scott Carey wrote:
> You must either increase the memory that ZFS uses, or increase
> Postgresql shard_mem and work_mem to get the aggregate of the two to
> use more RAM.
>
> I believe, that you have not told ZFS to reserve 8GB, but rather told
> it to limit itself to 8GB.
That is correct, but since it will use the whole 8 GB anyway, I can just
as easily say that it will reseve that memory ;)
>
> Some comments below:
>
> On Thu, Oct 30, 2008 at 8:15 AM, Christiaan Willemsen
> <cwillemsen(at)technocon(dot)com <mailto:cwillemsen(at)technocon(dot)com>> wrote:
>
> Hi there,
>
> I configured OpenSolaris on our OpenSolaris Machine. Specs:
>
> 2x Quad 2.6 Ghz Xeon
> 64 GB of memory
> 16x 15k5 SAS
>
> If you do much writing, and even moreso with ZFS, it is critical to
> put the WAL log on a different ZFS volume (and perhaps disks) than the
> data and indexes.
I already did that. I also have a separate disk pair for the ZFS intent log.

> Are you counting both the memory used by postgres and the memory used
> by the ZFS ARC cache? It is the combination you are interested in,
> and performance will be better if it is biased towards one being a
> good chunk larger than the other. In my experience, if you are doing
> more writes, a larger file system cache is better, if you are doing
> reads, a larger postgres cache is better (the overhead of calling
> read() in 8k chunks to the os, even if it is cached, causes CPU use to
> increase).
No, the figure I gave is this is without the ARC cache.
> If you do very large aggregates, you may need even 1GB on work_mem.
> However, a setting that high would require very careful tuning and
> reduction of space used by shared_buffers and the ZFS ARC. Its
> dangerous since each connection with a large aggregate or sort may
> consume a lot of memory.
Well, some taks may need a lot, but I guess most wil do fine with the
settings we used right now.

So It looks like I can tune the ARC to use more memory, and also
increase shared_mem to let postgres cache more tables?