Re: Swap space requirements on 10g installs - must it really equal RAM?

Hi Mark,

As you said since Oracle recommends 2x physical RAM anyhow, discussion is more academic and I am also interested to hear some other opinions.

If you use DISM segment, then DISM segments must be backed up by swap space. DISM pages are not locked automatically and because of that swap space must be allocated for the shared memory segment. This is a reason why I said:
"since SGA_MAX_SIZE is 10GB, you need 10GB swap space + enough for all other processes" - this is a minimum and this minimum is usually 1:1 mapping between physical RAM and allocated swap space (unless you set a SGA_MAX_SIZE much higher than your physical RAM). But only allocated disk space. On the other hand I think there is no 1:1 mapping of physical RAM consumed and physical disk/swap consumed. Consumed, not allocated. Someone can correct me here if I am wrong. If you allocate a 2x RAM swap space, the only cost here is the wasted swap disk space. Since a prices per disk unit are going down, wasting a 32G, 64G or 128G of disk space is not a big issue for any shop I think.

On the other hand, Since Solaris 2.6, swap space is not required to back the ISM segment since the memory is locked. Locked pages can not be swapped to disk since they must be allocated from physical memory and they can not be paged out by OS. In this case you can save a 10G of swap space and the rule of 1:1 mapping/allocation between a swap and physical RAM may apply only to the rest of (Total RAM - 10G).

IMHO. Cheers,

Goran

On 10/5/05, Bobak, Mark <Mark.Bobak_at_il.proquest.com> wrote:
> Hi Goran,
>
> This is not my area of expertise, so, bear with me here.....
>
> You say: "I think your sysadmin is right....there's no one-to-one
> mapping of physical RAM consumed and physical (disk) swap consumed."
>
> But then later you say: "In your case is SGA_MAX_SIZE 10GB, i.e minimum
> size of your swap space need to be 10 gigabytes plus the swap space
> reserved by all other processes on your system."
>
> Um, ok.....so, since SGA_MAX_SIZE is 10GB, I need 10GB swap space +
> enough for all other processes (mostly, Oracle server processes).
>
> This sounds suspiciously like physical RAM is backed with swap space, 1
> to 1. Based on this, if you wanted to be able to utilize all (or nearly
> all) your physical RAM, there would be no getting around a minimum of
> swap space equal to physcial RAM.
>
> Since Oracle recommends 2x physical RAM anyhow, I suppose the point is
> academic, but, I'm just trying to better understand how Solaris really
> works.
>
> Thanks,
>
> -Mark

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