Saturday, December 14, 2013

Swap from SAN

www.unixbabuforum.inI have to increase swap space on one of the servers. App teams are requesting to increase the swap size by approximately 50GB, but unfortunately we don't have enough free space in the internal disks to configure swap. So, we decided to configure swap from SAN Luns (Symmetrix). I would like to know how reliable the setup will be and how can we setup? 

We have powerpath configured, so I would like to use emcpower1a and emcpower2a for swap and each is 32GB. 

1. Is it reliable to configure swap on SAN? 
2. Can we configure luns using SVM or Veritas? Which is better? 


www.unixbabuforum.inI guess they will keep the SWAP on internal disks, if the SAN crashes the system will slow down, but the Solaris should not crash at all. 

I have tested right now with a Solaris Machine with an half of swap on local disk and an half on SAN. 
Removing the SAN connection the system continue to go without any kernel panic (now it is 5 minutes that is going). 

The machine was running a test environment with heavily used Oracle DB. 

Now I think they could have problems if the cache on the HBA is full. 

For sure it is not an optimal solution for a production system, I think I share your same feeling, but I must also say that we have several customers booting big SUN clusters from SAN, in this case if there is a failure on the SAN Level (SAN Switch, HBA, SAN Processor) the situation could be even worse. 


www.unixbabuforum.inIn my experience with SAN and Solaris there are just a couple of examples in which we adopted this strategy. We prefer to have swap and temporary data on local disks (as you probably would like to do having disk space). 
In terms of reliability you shouldn't have any problem, you have a symmetrix with a multipath, so the possibility the system crashes is very far from reality. The problem I see is in maintenance phase of the SAN, but I think you will maintain the swap on the internal disks, so in this case at least the machine will not crash. 

As far as I remember, it is possible to add swap in Solaris over a Veritas (http://slashzeroconf.wordpress.com/2008/07/17/addi ng-swap-space-with-vxvm-in-solaris/). 

A last consideration that could save you from future problems: consider to create the LUN on a RAID 10, consider to take in the symmetrix the smallest and fastest disks to reduce performance issues (for this purpose we use ONLY 73 Gb Disks FC 15K RPM). If you are already using a lot of swap, do not use raid groups that are already in use, but ask to the SAN admin to create one separated and to use the SP with lowest resource usage. 
By experience, swap can be a big problem for SAN if the load on SPs is not balanced and if you are using the wrong configuration on disks. 



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