questions from FreeBSD user

Matthew Dillon dillon at
Sun Jul 15 17:43:01 PDT 2012

:On Sun, Jul 15, 2012 at 5:02 PM, Wojciech Puchar
:<wojtek at> wrote:
:> i have few questions. i am currently using FreeBSD, dragonfly was just
:> tried.
:> 1) why on amd64 platform swapcache is said to be limited to 512GB? actually
:> it may be real limit on larger setup with more than one SSD.

    It seemed like a reasonable limit for the KVM overhead involved,
    though I don't remember the exact reason I chose it originally.

    The practical limitation for swap is 4096GB (4TB) due to the use
    of 32 bit block numbers coupled with internal arithmatic overflows
    in the swap algorithms which eats another 2 bits.

    We do not want to increase the size of the radix tree element because
    the larger structure size would double the per-swap-block physical memory
    overhead, and physical memory overhead is already fairly significant...
    around  1MB of physical memory is needed per 1GB of swap.

    There are a maximum of 4 swap devices (w/512GB limit by default in total,
    with the per-device limit 1/4 of that).  Devices are automatically
    interleaved and can be added and removed on the fly.  The maximum can
    be increased with a kernel rebuild but it is not recommended... you
    generally won't get more performance once you get past 4 devices.

:> 2) it is said that you are limited to cache about 400000 inodes unless you
:> use sysctl setting vfs.hammer.doublebuffer or so.
:> in the same time it is said to be able to cache any filesystem.
:> Can UFS be cached efficiently with millions of files?

    32 bit systems will be limited to ~100,000 inodes are so.

    64 bit systems calculated a default limit (kern.maxvnodes) based on
    available ram, with no cap.  So values > 1 million will be common.

    And you can always raise this value via the sysctl.

    UFS ought to be be cached by swapcache but there's no point using it
    on DragonFly.  You should use HAMMER.

:> 3) how about reboots? From my understanding reboot, even clean, means losing
:> ALL cached data. am i right?

    All swapcache-cache data is lost on reboot.

:> In spite of HAMMER being far far far better implementation of filesystem
:> that ZFS, i don't want to use any of them for the same reasons.
:> UFS is safe.

    A large, full UFS filesystem can take hours to fsck, meaning that a
    crash/reboot of the system could end up not coming back on line for
    a long, long time.  On 32-bit systems the UFS fsck can even run the
    system out of memory and not be able to complete.  On 64-bit systems
    this won't happen but the system can still end up paging heavily
    depending on how much ram it has.

    In contrast, HAMMER is instant-up and has no significant physical
    memory limitations (very large HAMMER filesystems can run on systems
    with small amounts of memory).

:> 4) will virtualbox or qemu-kvm or similar tool be ported ever to DragonFly?
:> i am not fan of virtualizing everything, which is pure marketing nonsense,
:> but i do some virtualize few windows sessions on server.
:> thanks

    With some work, people have had mixed results, but DragonFly is designed
    to run on actual hardware and not under virtualization.

					Matthew Dillon 
					<dillon at>

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