Relative performance of swap-backed MFS vs. regular UFS?

From: Clifton Royston (cliftonr_at_lava.net)
Date: 10/23/04

  • Next message: Erik Udo: "label editing" 
    Date: Fri, 22 Oct 2004 12:32:40 -1000
To: hackers@freebsd.org

      I have seen some conflicting information posted about this in the
    past, and I figure this is the best place to get an authoritative
    answer.

      For a large temporary file system which must hold short-lived files,
    mostly small but occasionally several very large ones (e.g. 100MB+), is
    it better for performance and stability if this file system:

      1) resides on a swap-backed MFS and trusts the OS to swap out
    low-priority blocks if needed under RAM pressure, or

      2) on a regular UFS and trusts the OS to buffer as many blocks as
    possible into RAM when RAM is free?

      The application here is amavisd-new, which needs transient space for
    unpacking and virus-checking mail files in transit. It is designed
    such that the files never need to exist permanently; it's just fine if
    everything vanishes on a reboot.

      Each scanning server has about 1GB of RAM, which is mostly free most
    of the time. I initially had /var/amavis a regular directory under the
    /var fs; then I switched to a 128MB MFS system backed by a 1GB swap on
    /dev/ad0s1b. This works 99.99% of the time, and seemed to run a little
    faster, but occasionally a couple big ZIP files can come through at
    once and run it out of space.

      I temporarily enlarged it to 256MB which is working, but as I worked
    out the worst case scenario, I realized it really would need to be
    nearly 1GB to handle multiple zip-bombs each hitting the 100MB size
    limit. This makes me wonder if it's wise to specify a 1GB MFS on a
    system with only 1GB RAM, or wiser to just revert to a regular file
    system?

      -- Clifton 

    -- 
          Clifton Royston  --  cliftonr@tikitechnologies.com 
         Tiki Technologies Lead Programmer/Software Architect
Did you ever fly a kite in bed?  Did you ever walk with ten cats on your head?
  Did you ever milk this kind of cow?  Well we can do it.  We know how.
If you never did, you should.  These things are fun, and fun is good.
                                                                 -- Dr. Seuss
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