maxdsiz kernel parm, swap, phys memory

[Jeff Patrick]

Fellow UNIX Sysadmins,

What do you recommend for the swap size on HP9000 servers? i.e. 1GB physical memory, what would you recommend for swap size on an Oracle database server (HP-UX 11, Oracle 7.3.4.4). I've heard conflicting information from HP, Oracle, etc. Also, what can you tell me about pseudo swap (I think the 'swap_mem_on' kernel parameter)? Also, my swchunk parm is 2048 and maxswapchunks is 4096.

Recently, we changed our 'maxdsiz' parm from 64MB to 128MB, and now our Oracle snapshots are causing memory utilization to skyrocket, and in some cases, using up all our swap as well. Any ideas/recommendations/suggestions? Thanks!!!

[Bruce Regittko_1]

Hi,

Typically when a process is created, it allocates (reserves) a certain amount of swap space. The problem is, if there is a relatively small amount of swap compared to physical memory, you may "run out" of swap despite that fact that there is plenty of memory and/or the processes would be unlikely to need swap in the first place.

To get around this, you can enable pseudo swap (the default, I think). This allows processes to reserve more swap space than is actually present. The amount of "overbooking" can be up to 75% of physical memory. Thus for a system with 4GB of memory and 2GB of actual swap, with pseudo swap enabled, processes can reserve up to 5GB of swap - 2GB actual + 3GB (75% of 4GB).

HP recommends a minimum amount of swap equal to physical memory and claims typical systems will have 2 to 4 times configured. See their book HP System Administration Tasks available online at docs.hp.com for more information.

--Bruce

[Madhu Sudhan_1]

Regarding "swapmem_on", This parameter tells the OS to use free RAM as a swap in addition to the device/primary swap which is enabled by default. I think the following URL best explains about swapmem_on.

http://docs.hp.com//hpux/onlinedocs/os/KCparam.SwapMemOn.html

Hope this helps.

...Madhu

[CHRIS_ANORUO · Accepted solution]

Hi Jeff,

The explanation by Bruce is to the point on pseudo swapping, you can set swapmem_on=1, nbuf and bufpages=0 to get dynamic buffer caching and swapping. I have attached my servers kernel configuration file. I have 10.20 and oracle 8.0.5 running, disk swap is 2gb and Phy mem is 1gb. Check the shared memory settings and the maxdsiz value from the attached file.

[Rhonda Thorne]

Jeff....

Shared Memory values should be as large as ech instance or Oracle SGA.....

maxdsize is set to a value that is required by oracle. Orace will state in metaink the required maxdsize minimum according to the volume of SGA and number of instaces of Oracle on the system.

Good luck