32GB limit per swap device?

Jeremy Chadwick freebsd at jdc.parodius.com
Wed Aug 10 13:21:51 UTC 2011


On Wed, Aug 10, 2011 at 01:42:11PM +0300, Daniel Kalchev wrote:
> 
> 
> On 10.08.11 11:47, Jeremy Chadwick wrote:
> >So we're back to where we started: swap slices/partitions can be
> >greater than 32GBytes in size, but "something" is limiting the
> >maximum amount of memory which can be dumped to a single swap swap
> >to 32GBytes.
> It seems there is still some confusion. Partition size(s) is 64GB,
> but FreeBSD will use only 32GB of that for swap.
> 
> Trying:
> 
> sysctl debug.minidump=0
> sysctl debug.kdb.panic=1
> 
> Produces 64GB dump, but.. I can't see the dump on the swap device
> (savecore doesn't find the magic number). My swap device is gmirror
> or two slices (/dev/mirror/swap).
> 
> Removing the gmirror and listing both slices for swap .. works.
> 
> 
> I could use separate device for dumps, but the disk real estate on
> this particular blade is limited...
> 
> Well, I really asked two questions:
> Q1: Is FreeBSD still limited to 32GB per swap slice?
> A1: yes. There is limit set in /usr/src/sys/vm/swap_pager.c
> 
> Q2: If FreeBSD will only use 32GB of the slice for swap, will it
> dump larger (64GB in my case) core in there?
> A2a: yes.
> A2b: If the swap slice is gmirrored, you don't get any dump back.
> 
> I believe the gmirror bug might exist in smaller partitions as well,
> but haven't tested it yet (have few such systems that never duped
> core). It does not matter if I do full dump or minidump: on
> gmirrored 64GB partittion savecore does not find anything.

Is this somehow related to the NOTES section of gmirror(1), which talks
about caveats/problems with generating kernel dumps on gmirrored swap?

It seems that as the more technical details of your system start to
present themselves, the answers start to present themselves as well.

-- 
| Jeremy Chadwick                                jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                   Mountain View, CA, US |
| Making life hard for others since 1977.               PGP 4BD6C0CB |



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