Questions re swap-on-zfs

Tue Dec 3 08:16:34 UTC 2013

Hello there :)

I had similar issues with a test box (FreeBSD 9.1) I used few months ago 
with not so much free RAM - the box had 1-3GB total RAM as far as I 
remember.
After digging I have found that the problem was actually a swap deadlock 
:)
What I mean: when you heavily utilize the system, for example building 
ports or something, the system tries to swap some memory, used by the ZFS. 
The problem is, that the swap is actually on the ZFS itself and uses the 
same memory. 
I am not sure I describe this precisely, but the overall result is that 
the system uses more and more swap space just to be able to swap something 
relatively small. It ends up crashing the system several times.

What I did and made the things OK - I just removed the swap from the ZFS. 
I used the first/last 1-2GB on each disk out of the ZFS just for swap - 
the system had 3 disks. 
After initiating the same operations I have noticed that the system used 
less that 0.5GB swap, so the problem was not the quantity, but the fact 
that swap is on ZFS. Maybe on a system with more RAM that will not be an 
issue :)

Regards,

Ivailo TANUSHEFF
Head of IT Operations Department

Raiffeisenbank (Bulgaria) EAD
55 Nikola Vaptzarov Blvd , Expo 2000, Building E, 1407 Sofia, Bulgaria
Tel.: +359 2 8610-816
Fax: +359 2 9628350
e-mail: Ivailo.TANUSHEFF at raiffeisen.bg
web: www.rbb.bg

From:
Beeblebrox <zaphod at berentweb.com>
To:
freebsd-fs at freebsd.org, 
Date:
03.12.2013 09:03
Subject:
Re: Questions re swap-on-zfs
Sent by:
owner-freebsd-fs at freebsd.org

Thanks Adam,

I re-shuffled my HDD and extracted the swap on the spindle HDD to a 4G
partition.
A poudriere run on all 4 cores produced an "out of swap space" message 
soon
after. I first thought it must be some mistake, but later saw swap usage 
max
to 1.9G when poudriere dropped to 1 build jail and was only building
libreoffice. I can imagine what swap gets to when building 4 large ports
concurrently then.

It seems like code is a little larger in 11. I mean, root "/" and /usr
occupy more HDD space and I never ran out of swap before. 4G ram + 4G swap
would be more than enough I assumed. I'll probably bump up to 6G swap 
space
(4 cores x 1.5G = 6G)

Thanks to everyone for their input.

-----
FreeBSD-11-current_amd64_root-on-zfs_RadeonKMS
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