[pve-devel] [Bug 1484] RFC: Improve swap support (zswap, zram & etc)

Eneko Lacunza elacunza at binovo.es
Tue Aug 22 11:58:54 CEST 2017


Hi Timofey,

El 22/08/17 a las 11:45, Timofey Titovets escribió:
> Hi list,
> By suggestion from: Fabian Grünbichler,
> i try open topic on the dev list.
>
> So, my suggestion (copy paste from issue):
> since 3.11 kernel Linux support zswap, and since 3.15 kernel linux support zram.
>
> (i'm a creator of https://github.com/Nefelim4ag/systemd-swap)
> What i can suggest (That just IMHO):
>
> - Enabling Zswap along with z3fold in kernel build, can make life
> easier for your customers
>    Zswap is a compressed cache between RAM and SWAP device
>    z3fold it's a successor of zbud and can compress 3 pages in 1, in
> compare to zbud 2 in 1 (z3fold disabled by default)
>
> - Also that's will be cool if you add some info in Host status about
> zswap efficiency
>    i.e. how many swap stored in zswap in compare to physical swap devices
>   (i'm already made that, so you can try do something like that:
>    https://github.com/Nefelim4ag/systemd-swap/commit/83142e4903df02df4498f782cdaabda72f43b491)
>
> Also you can adopt systemd-swap if you wish, like you do with ksmtuned
>
> So i glad to hear some opinion from proxmox dev's,
> because you can know your customer cases,
I'm not a Proxmox developer myself, but didn't know these technologies, 
which are quite interesting.

I have found problems in the past with Linux kernel swaping too much 
under cache pressure, causing severe performance problems; what I have 
done on those cases is to turn off swap. Also on newer installations I 
only set an small swapping area, usually 1GB instead of the 4GB that 
creates proxmox installer. On each new installation I think I really 
could turn off swap altogether... :) Once you start swapping, a 
virtualization server's performance degrades too much to be usefull at 
all on my experience.

I see that in wikipedia zram and zswap articles refer to low resource 
computers; would those technologies be really usefull for a 
virtualization server?

Thanks
Eneko

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