Date: Mon, 19 Aug 2002 09:04:13 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: freebsd-stable@FreeBSD.ORG Subject: Re: root (/) not soft-updates by default ? Message-ID: <200208191604.g7JG4DOU077232@apollo.backplane.com> References: <20020819144928.GA6628@nebula.wanadoo.fr> <20020819105452.A14530@blackhelicopters.org> <m3ofbylv71.fsf@merlin.emma.line.org>
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This conversation is moot, since modern IDE drives (and apparently seeping
into SCSI drives as well) do track-at-once writes to the physical media,
there is no ordering guarentee no matter what you do, and IDE performance
with write caching turned off sucks so badly you pretty much *have* to turn
it on if tags are not supported or suffer penalties so severe that the last
time we tried it our user community was up in arms over the result.
So short of caching the data in NVRAM you are *screwed* no matter what
you do. Oh, wait, even caching the data in NVRAM doesn't save us, since
track-at-once writes rewrite sectors we never modified. So we might as
well turn write caching on. If you don't like the default, you can turn
it off in /boot/loader.conf.
In any case, Jaime Bozza's explanation (the smallish root can fill up with
softupdates enabled when installing a new system over the old one) is the
correct one. This issue is fixed in -current's softupdates but not
fixed in -stable's. The business about IDE not ordering writes is also
correct, but the statement that softupdates is somehow *worse* then a
normal mount with an IDE disk is not correct. Both are equally dangerous
due to the track-at-once writes which IDE drives tend to do these days.
Since very little on the root partition is modified once a system has
come up, not enabling softupdates on root is not a big deal.
-Matt
Matthew Dillon
<dillon@backplane.com>
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