Re: Swap partition priority

Short answer, yes - interleaving is hardcoded and the swapon(2) syscall only accepts the device node name (or file name). Here’s the doc header comment for the allocation code in sys/vm/swap_pager.c:

    915 /*
    916  * SWP_PAGER_GETSWAPSPACE() -   allocate raw swap space
    917  *
    918  *      Allocate swap for up to the requested number of pages.  The
    919  *      starting swap block number (a page index) is returned or
    920  *      SWAPBLK_NONE if the allocation failed.
    921  *
    922  *      Also has the side effect of advising that somebody made a mistake
    923  *      when they configured swap and didn't configure enough.
    924  *
    925  *      This routine may not sleep.
    926  *
    927  *      We allocate in round-robin fashion from the configured devices.
    928  */

However, the suggestion to use a concatenated GEOM should work great because that’s a single device according to the swap and the GEOM allocator is (should be) entirely transparent to the swap strategy:

Relevant snippet from swapgeom_strategy:

   3212         bp->b_caller1 = bio;
   3213         bio->bio_caller1 = sp;
   3214         bio->bio_caller2 = bp;
   3215         bio->bio_cmd = bp->b_iocmd;
   3216         bio->bio_offset = (bp->b_blkno - sp->sw_first) * PAGE_SIZE;
   3217         bio->bio_length = bp->b_bcount;
   3218         bio->bio_done = swapgeom_done;
   3219         bio->bio_flags |= BIO_SWAP;
   3220         if (!buf_mapped(bp)) {
   3221                 bio->bio_ma = bp->b_pages;
   3222                 bio->bio_data = unmapped_buf;
   3223                 bio->bio_ma_offset = (vm_offset_t)bp->b_offset & PAGE_MASK;
   3224                 bio->bio_ma_n = bp->b_npages;
   3225                 bio->bio_flags |= BIO_UNMAPPED;
   3226         } else {
   3227                 bio->bio_data = bp->b_data;
   3228                 bio->bio_ma = NULL;
   3229         }
   3230         g_io_request(bio, cp);
   3231         return;


> On Aug 18, 2026, at 9:00 AM, Joshua Armstrong <[email protected]> wrote:
> 
> I’ll throw my hat into this ring.
> 
> As soon as I’m finished with my meeting, I’ll check the swap management / virtual memory code to see if interleaving is hardcoded or not. I believe the current (and long-standing, according to my ancient copy of the Unix System Administration Handbook) behavior is to interleave swap writes between configured devices for performance.
> 
> I agree, however, that having a priority system would make more sense in today’s world. I sometimes end up creating a temporary zvol and using that as emergency extra swap if I’m building world, but I would assume this has a performance penalty (though perhaps not massive) versus swap partitions on bare disks.
> 
> -Joshua
> 
>> On Aug 18, 2026, at 3:48 AM, Miroslav Lachman <[email protected]> wrote:
>> 
>> On 18/08/2026 09:49, Stéphane Rochoy wrote:
>>> Andrea Venturoli <[email protected]> writes:
>>>> Is it possible to set swap partition priority?
>>>> According to swapon(8), the answer is no, but I wonder if the man page
>>>> might be outdated, if patches are available, etc...
>>>> 
>>>> The reason is I have SSDs and HDDs in the same system: I normally use
>>>> swap on SSDs (for speed obviously), but sometimes the amount is not enough.
>>>> So I'm ponderind adding swap on HDDS too, but this would slow down
>>>> everyrting unless it's really used as last resort.
>>> Hello,
>>> Long ago I came accross a setup were the swap device was a GEOM mirror
>>> (see gmirror(8)). AFAIK it was working except for vmcores: the kernel
>>> was always using the first device which required to help savecore(8) a
>>> bit.
>>> So maybe there's some GEOM class matching your needs.
>> There is geom_concat - see man gconcat. It may be possible to use it for swap (never tried it).
>> 
>> Best regards
>> Miroslav Lachman
>> 
>