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overlapping swap

I recently saw a system where they had swap on /tmp (as in a worstation) but had then added c0t0d0s1 as a swap partition. I said they were in error and that they could have as many swap partitions as they like but not in conjunction with swap on /tmp. But I can find no reference to this. Has anyone else come across this ?

geotiger

There are good and bad about using /tmp as part of swap. The good thing about it is that you are more efficiently using /tmp space. The very bad thing about it is that if you have only /tmp as your swap space, the system will be hung or crashed if users used all the /tmp space.

As long as there is (are) other partition(s) added to swap list, it is ok to use /tmp as swap space.

nicob

Hi,

It is normal behaviour for unix to use /tmp as temporary directory and as swap. You can afterwards configure more swap partitions, but the first swap you configure will always be used for both swap and /tmp. This has nothing to do with if it is a workstation or a server, it is a unix standard.

This is also why you have to configure swap/tmp with a 2 x memory size. This will prevent swap/tmp from filling up quickly. If you have a system with 500Mb of memory then you will have to make a /tmp filesystem of 1Gb, this because it can then use /tmp to dump its total memorysize incase the system crashes and does a crashdump.

So you will see that when you do a df -k on a unix system (solaris in this case) then you will get the following output :

nicob@sis250% df -k
Filesystem kb used avail capacity
swap 2791608 1024 2790584 1% /tmp

nicob@sis250% swap -l
swapfile dev swaplo blocks free
/dev/dsk/c0t0d0s1 32,1 16 4197856 4197856

Hope this answers the question.

Regards,

Nico.

mark.dudley

ASKER

mounting swap on /tmp does not give you your first swap space, as I understand it. It says you should use virtual memory for /tmp - giving less swap space.
On reflection I dont think this partition is likely to have caused the error I saw.

geotiger

What error? The one danger (sometime fatal) is running out of space in /tmp if you have /tmp as your only swap space since users can write to /tmp. Other than that, /tmp should act as usual as other partitions for swap.

ASKER CERTIFIED SOLUTION

klmorris052500

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klmorris052500

Also, to add additional swap to the system, create a file as indicated above, and put this in your /etc/vfstab:

<fully qualified path to new file> - - swap - no -