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Swap space on the SAN. Any problems?

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James Brand
Frequent Advisor

Swap space on the SAN. Any problems?

N Class server running HP-UX 11.11. I need to increase swap and no space left on the OS disk. So I either get a larger boot drive or allocate swap on the SAN, which is EMC Symetrix via two Fibre channel adapters. The server is running EMC PowerPath.

This server never swaps and will certainly never page to secondary swap space.

Has anyone ever had problems with either device or filesystem swap on SAN? I used filesystem swap on Jamacia disk years ago without problems. Any feedback is welcome.

Thanks,
Jim
8 REPLIES
Patrick Wallek
Honored Contributor
Solution

Re: Swap space on the SAN. Any problems?

There should be no issues. Having swap space on the SAN will be fine, but will be several GB worth of disk space that's not doing anything.
sujit kumar singh
Honored Contributor

Re: Swap space on the SAN. Any problems?

Hi Jmaes,


I do not think that shall be any sort of problems. I have myseld seen rp84XXX ORacle DB Servers 11.11 OS systems using EVA 8K LUNS assigned to them for swapping.

Just one thing to add , uou shall need to do two things additionally, you might be aware of that:
1)suppose you create a New VG for that then

must edit the /etc/lvmrc and put that VG entry in the custom_VG_activation so that the VG is activated at the boot time say vgXX.

2)might need to adjust the Kernel Tunables as for the New Swap Size on the system as maxswapchunks

3)must create the Swap LV on that VG with the command as

#lvcreate -L -C y -s y -r n -n /dev/vgXX

4)put the entry in the /etc/fstab to acivate swapping on that LV at the boot-time.

/dev/vgXX/ ... swap pri=1 0 0

5)do a swapon on that LV
#swapon -a (after adding that entry to /etc/fstab)
or
#swapon /dev/vgXX/


regards
sujit
Patrick Wallek
Honored Contributor

Re: Swap space on the SAN. Any problems?

>>must edit the /etc/lvmrc and put that VG
>> entry in the custom_VG_activation so that
>>the VG is activated at the boot time say
>>vgXX.

You should only need to do this if the server is part of an MC Service Guard cluster.

Otherwise ALL VG's should be activated automatically.

# grep AUTO_VG_ACTIVATE /etc/lvmrc
AUTO_VG_ACTIVATE=1

Run the 'grep' command above to check your /etc/lvmrc. If AUTO_VG_ACTIVATE is set to '1' then ALL VG's will be activated automatically at boot time. No script modification is necessary.

However, if AUTO_VG_ACTIVATE is set to 0, then you will need to add the VG for your swap to the list of VGs that need to be activated, as stated above.
Steven E. Protter
Exalted Contributor

Re: Swap space on the SAN. Any problems?

Shalom,

swap space on the SAN.

Definitely something you can do.

Definitely something I would never do.

Someone pulls or cuts a cable and the system is messed beyond belief.

Doing it through a SCSI cable to a Jamaica box is something I have often done. You get the same throughput as you do to an internal disk.

Based on the conditions you lay out, if the SAN and LUNS are reliable you can do this, but you need to watch throughput.

The LUN should not be RAID 5, that will slow down disk writes.

SEP
Steven E Protter
Owner of ISN Corporation
http://isnamerica.com
http://hpuxconsulting.com
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Founder http://newdatacloud.com
TTr
Honored Contributor

Re: Swap space on the SAN. Any problems?

> This server never swaps and will certainly never page to secondary swap space.

If you have the luxury and the SAN space is there you can use it otherwise larger boot disks or an extra pair of local disks might be cheaper.
James Brand
Frequent Advisor

Re: Swap space on the SAN. Any problems?

We schedule one outage a year on this server so I hate to take it down just to install a bigger OS drive.

The risks of losing SAN connection are justified. But if that happens we are in big trouble anyway and a swap device, especially one that is not swapping, might be the least of our worries.

That said, I realize that the HP-UX I/O subsystem seems fairly immune from momentary disk interruptions. Might not be so resilient for swap space if the server can't see that for a few seconds.

I appreciate everyones responses!
Patrick Wallek
Honored Contributor

Re: Swap space on the SAN. Any problems?

>>Someone pulls or cuts a cable and the system
>>is messed beyond belief.

Not if it is probably set up with alternate paths.

>>The LUN should not be RAID 5, that will
>>slow down disk writes.

Baloney!!!!! Modern arrays are fast enough that I seriously doubt there is a significant impact between RAID 1 and RAID 5. That is why you have large amounts of cache on the controllers and fast processors on the raid controllers.

Besides...

>>This server never swaps and will certainly
>>never page to secondary swap space.

So that makes the above comment about RAID 5 moot.

And to quote/paraphrase our good, albeit MIA, friend A. Clay Stephenson: "Worrying about performance when setting up swap is like rearranging the deck chairs on the Titanic." It's pointless. If you are swapping to any significant extent your performance will ***SUCK*** anyway.
James Brand
Frequent Advisor

Re: Swap space on the SAN. Any problems?

Thanks to all.