swap on network ?

[Maaz]

in SUSE Linux Enterprise 11, swap can be on network(NFS), they (Novell/SUSE) claims that it will prevent crashes, and boost reliability.

How ?

I think its bad because
1, if the Swap is on network.. then I/O increases
2, what if network for any reason goes down for 3 seconds only, and at the same time SUSE tries to "so" and/or "si" I mean tries to paging.

whats your opinion ?
Regards

[Ivan Ferreira]

Right now I disagree. Do you have that documentation? Are you sure is talking about swap and not kdump?

[Robert Walker_8 · Accepted solution]

Maaz,

I wouldnt think swap was good to place on the network. These days with larger memory systems swap probably doesnt need to be as large as it used to be - depending on workloads etc.

Disks are cheap unless your using SSD as a boot disk, but then if your doing that then you likely have SAN as well which is more reliable than standard networking.

Theres a difference in what can be done and what should be done. Most software will likely say you can run on 256MB ram, but mostof us know that is the absolute bare minimum when reading specs like that.

As mentioned earlier losing swap at any time will likely cause system panic and NFS is not a reliable transfer mechanism - especially if using it via the TCP/IP UDP network protocol.

Robert

[Maaz]

"SUSE 11 includes enhanced virtualization optimization with the Microsoft Hyper-V, VMware and Xen hypervisors, a faster package stack for updating applications and the ability to swap memory remotely over the network to prevent crashes, boosting reliability."

Source:
http://searchenterpriselinux.techtarget.com/news/article/0,289142,sid39_gci1351714,00.html?track=NL-306&ad=696996&asrc=EM_USC_6409916

---
http://www.novell.com/linux/whats_new_top.html
http://www.novell.com/products/server/reliable.html

Regards

[Maaz]

"By using swap over NFS, you can protect your systems against application restarts and expensive downtime."

Source:
http://www.novell.com/products/server/reliable.html

[Huc_1]

Yes read your 3 link's and still think this is not such a good ideas, the 'system's'' use the network as just an other bus so having the swap using nfs to have this mounted remote seems 'normal', or a way to go.

but!

You need a very reliable network (probably dedicated and simple one, in my opinion)

You also need a 'system of some kind to export our swap from'... you have to maintain this!

You still have to have some disk block reserved for swap, so you do not gain anything there !

on the other hand.

to have your swap local is simple and mostly once set up correctly, mostly you can just forget about it, and with today cheaper disk, you could/can save some space on an other disk for the case where your swap disk fails and you need to reboot quickly, this is probably simpler then fixing the network, nfs or the other system that host the disk that host the swap file .

I do not mind being proved wrong... , but for now will stick to local swap.

enjoy life.

Jean-Pierre Huc

[Matti_Kurkela]

The swap-over-NFS functionality is marketed within the context of diskless servers.

There seems to be two claims:
1.) diskless servers are cheaper

2.) *on diskless servers*, swap over NFS is more reliable (compared to the alternative of no swap at all, NOT compared to a traditional server with swap on local disks)

The justification for swap-over-NFS feature (in marketing-speak):

"Once you are no longer restricted to local storage for swap space, you can cut costs dramatically. You can use less costly diskless servers, and simplify server administrationâ thus reducing acquisition, implementation, administration, and management costs."

Source:
http://www.novell.com/products/server/reliable.html

When marketing claims that "something is better", always ask yourself "better than _what_?" The chosen comparision is not always the one you'd assume.

MK