Date: Wed, 30 Jan 2002 14:14:38 -0500 From: Brian T.Schellenberger <bts@babbleon.org> To: Espen Tagestad <espen@modula.no>, questions@FreeBSD.ORG Subject: Re: Softupdates Message-ID: <20020130191439.323464078@i8k.babbleon.org> In-Reply-To: <20020130124756.A15728@modula.no> References: <3C593C4A@epostleser.online.no> <20020130124756.A15728@modula.no>
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On Wednesday 30 January 2002 06:47 am, Espen Tagestad wrote:
> On Wed, Jan 30, 2002 at 12:34:11PM +0100, Christer Gundersen wrote:
> > As i read the RELEASE NOTES, i see that SOFTUPDATES will be activated by
> > default under install. but it also says that it will not enable
> > SOFTUPDATES on the root ( / ) fs . why? is that bad?
>
> Because with SoftUpdates read/write operations often delay, sometimes
> up to 30 seconds before it's done. Then, if a machine crash occurs it
> may lead to a larger loss of data. That can cause unrecoverable damage
> to your system.
As long as you turn off write caching, this isn't true (if it were true, then
softupdates would be a disaster for *all* filesystems, not just root). That
is, things may be delayed but they are in a consistent state at all times,
and never corrupt. At worst, the last files you deleted will "undelete"
themselves and the last files you created may not be there, but no
file-system corruption will occur.
Now it IS a bad idea (even though many people do it without happening to get
burned) to run with write caching *and* softupdates both on. In general,
it's a bad isea to run with write caching, period, but combining it with
softupdates makes things a lot worse, but softupdates without write caching
is safer than the other way 'round. Of course, turning both off is safest
and slowest, so pick your poison. I use softupdates on ALL file systems and
turn off write caching myself.
(To turn off write-caching, put this in /boot/loader.conf:
# write cache considered dangerous
hw.ata.wc=0
)
The reason for the general advice to turn it off on / is because it does
introduce a delay and / is traditionally rather small. The delay effectively
gives you less space in a file system since freed space may not be freed yet,
but if you make / larger than usual and turn on softupdates you'll get the
speed benefit of softupdates and yet won't risk running out of disk space.
(It seems an especial shame to turn off softupdates on whatever file system
contains /tmp since the benefits are larger on a file system with lots of
writes.)
>
>
> regards,
>
> Espen Tagestad
>
>
>
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