Swap on ZFS

Rolf Nielsen rmg1970swe at gmail.com
Wed May 7 04:20:55 UTC 2014

Hash: SHA1

On 2014-05-07 02:56, Daniel Staal wrote:
> --As of May 7, 2014 2:25:47 AM +0200, Rolf Nielsen is alleged to
> have said:
>> On 2014-05-06 21:21, David Noel wrote:
>>> Is swap on ZFS still ill-advised? All the forum, list, and
>>> blog posts I find say it's a no-go. Is this still the case? The
>>> idea behind it not working is that ZFS needs memory to write to
>>> disk, so when you need to swap (are low on memory) ZFS won't be
>>> able to write.
>>> I found some talk of having a tunable added as a workaround
>>> that would reserve a certain amount of memory for ZFS so this
>>> wouldn't be a problem, but have no idea if anyone's made any
>>> progress towards implementing it.
>> Just out of curiosity, why do you want it?
>> To get swap on ZFS, you first need to create a ZFS filesystem on
>> one or more devices, then you create a dedicated volume inside
>> that filesystem and use that dedicated volume as swap. To me that
>> seems to add unnecessary complexity, similar to using a file
>> backed md device as swap. Please don't take this as criticism;
>> you may very well have good reasons for wanting to do this. I'm
>> just curious about those reasons.
> --As for the rest, it is mine.
> Because it's actually simpler than the alternative, in many cases.
> The creating a ZFS filesystem is 'free' in this - you are only
> planning on doing this if you are already running a ZFS-based
> system, so you're already creating the filesystem.  That leaves
> creating the dedicated volume inside that and using it as swap -
> which is as easy or easier than formatting and using a dedicated
> disk as swap.
> So, really using swap on ZFS is no harder or easier than using a 
> dedicated swap disk, and no more complex.  And if you *aren't*
> planning on a dedicated swap disk, it starts adding complexity: If
> you aren't using a dedicated swap disk, then you're probably
> sharing it with a disk that you'll be using in the ZFS filesystem -
> which means you now need to format and partition that disk, which
> you didn't need to do before.  You also have to monitor and
> remember that the disk is partitioned, if you ever have to replace
> it.  (Which otherwise ZFS would make easy - just swap in a new one,
> and tell ZFS to use it to replace the failed disk.)
> So your steps are: 1. Create ZFS filesystem. 2. Create swap inside
> filesystem. 3. Configure FreeBSD to use swap.
> vs. 1. Partition Disks. 2. Set up Swap partition. 3. Configure
> FreeBSD to use swap. 4. Create ZFS filesystem on other partition.
> Note of course that one of the points of using ZFS is the ease and 
> flexibility of creating volumes inside it - a ZFS user is probably 
> creating multiple at setup, and the swap volume isn't all that
> different to create. And again, you're giving up the ability to use
> ZFS to manage the device on the fly, which is one of ZFS's best
> benefits.
> Daniel T. Staal
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I'm not referring to the complexity of doing the setup, but to the
added layer (a volume on a filesystem on a disk vs. a partition on a
disk). I use ZFS for data storage, but I don't use any zvols. And my /
is on UFS on a "dangerously dedicated" 40GB SSD that also has the swap

If I want to talk to my mother, I call my mother and talk to her. I
don't call my sister and have her call my mother and relay everything.
And for the same reason, I don't see why I should put a filesystem or
swap on a volume on a filesystem.

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