32GB limit per swap device?

Chuck Swiger cswiger at mac.com
Tue Aug 9 14:37:30 UTC 2011

On Aug 9, 2011, at 7:26 AM, Daniel Kalchev wrote:
> I am trying to set up 64GB partitions for swap for a system that has 64GB of RAM (with the idea to dump kernel core etc). But, on 8-stable as of today I get:
> WARNING: reducing size to maximum of 67108864 blocks per swap unit
> Is there workaround for this limitation?

Apparently, the 32GB swapspace limit is per swap area; you can add up to 4 swap areas so create two or three 32GB swap partitions.


PS: Older thread with some specific details:

Begin forwarded message:
> From: Matthew Dillon <dillon at apollo.backplane.com>
> Date: August 5, 2008 11:29:21 AM PDT
> To: Max Laier <max at love2party.net>
> Cc: freebsd-stable at freebsd.org, Lin Jui-Nan Eric <ericlin at tamama.org>
> Subject: Re: Max size of one swap slice
> :> Recently we found that we can only allocate 32GB for one swap slice.
> :> Does there is any sysctl oid  or any kernel option to increase it? Why
> :> we have this restriction?
> :
> :this is a consequence of the data structure used to manage swap space.  See 
> :sys/blist.h for details.  It *seems* that you *might* be able to increase the 
> :coverage by decreasing BLIST_META_RADIX, but that's from a quick glance and 
> :most certainly not a good idea.
> :
> :However, the blist is a abstract enough API so that you can likely replace it 
> :with something that supports 64bit addresses (and thus 512*2^64 bytes of swap 
> :space per device) ... but I don't see why you'd want to do something like 
> :this.  Remember that you need memory to manage your swap space as well!
> :
> :-- 
> :/"\  Best regards,                      | mlaier at freebsd.org
> :\ /  Max Laier                          | ICQ #67774661
>    The core structures can handle 2 billion swap pages == 2TB of swap,
>    but the blist code hits arithmatic overflows if a single blist has
>    more then (0x40000000 / BLIST_META_RADIX) = 1G/16 = 64M swap blocks,
>    or 256GB.
>    I think the VM/BIO system had additional overflow issues due to
>    conversions back and forth between PAGE_SIZE and DEV_BSIZE which
>    further restricted the limit to 32GB.  Those restrictions may be gone
>    now that FreeBSD is using 64 bit block numbers, so you may be able to
>    pop it up to 256GB with virtually no effort (but you need to test it
>    significantly!).
>    With some work on the blist code only (not its structures) the arithmatic
>    overflow issues could also be resolved, increasing the swap capability
>    to 2TB.
>    I do not recommend changing any of the core blist structure, particularly
>    not BLIST_META_RADIX.  Just don't try :-).  You do NOT want to bump
>    the swap block number fields to 64 bits.
>    Also note that significant memory is used to manage that much swap.  It's
>    a factor of 1:16384 or so for the blist structures and probably about
>    the same amount for the vm_object tracking structures.  32G of swap needs
>    around 2-4MB of wired ram.
> 					-Matt
> 					Matthew Dillon 
> 					<dillon at backplane.com>
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