Swap size for large memory machines

[Steve DeJarnett]

In article <1115 at srhqla.SR.COM> mdh at srhqla.UUCP (Matt Hardin) writes:
>In article <1335 at esquire>  writes:
>>I used to feel (for reasons unknown) that twice as much swap space as
>>real memory was reasonable--but does that make any sense when you have
>>256 megabytes of memory?
>>I'm asking because the standard 'b' partition on Pyramid's layout for
>>the 1 gigabyte NEC 2363 drive is 30 megabytes.  That seems a bit stingy
>>and I'm wondering what to do about it.  Should I create my own (larger)
>>partitions (which I'm doing anyhow for other disks)?
>
>Don't change the partitioning for your system drive. The guy who installed
>our system (from Pyramid, no less) did that and it caused me nothing but grief
>when the first PTF tape came along and replaced conf.c. Blooey! There went our
>modified partitions! It also replaced /etc/disktab (I found out later), 
>causing disktab's version of the partitioning to revert back to normal.

	You can change your partitioning, but that's one more thing you have
to check every time you install a PTF or an upgrade.  Having been burned by
PTF's once (/dev/null was recreated as a 2 byte long file -- needless to say,
it grew rapidly :-(

	Therefore, the moral is:  ALWAYS check what the PTF does before 
installing it.  Look at all of the files it changes.  If it's changing
something that you think it has no business changing, stop and make a backup
copy of that file (they've also been known to replace /etc/rc.local :-(

>What I am doing now is using both the b and c partitions as swap by
>issuing a swapon command for each partition (really I just defined partitions
>b and c as swap in /etc/fstab, but it works out the same way). This gives us
>ample swap space for our machine. What took me by surprise was that you could
>issue two or more swapon commands! I get to keep my standard partitions and 
>have as much swap space as I need!

	If you have multiple drives, you also should spread the swap partitions
across the drives.  We have three Eagles with 3 b partitions used for swap (on
a 98x with 26 Meg of physical memory -- so much for 2x physical -- our swap is
closer to 3x physical).

	What it all boils down to is you need enough swap space to hold every
job you might conceivably have running at any time (well, that's a bit of an
overstatement, but you get the idea).  If all you do is compile-edit-run, you
can probably make do with 30 or 40 Meg of swap, provided you don't have 200
people doing these compile-edit-run jobs simultaneously.  On our system, it
usually works out to about 3 Meg of swap per user on the system.  Your mileage
will vary.

>				Matt Hardin
>				mdh at SR.COM

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