Date: Tue, 30 Apr 2002 10:10:02 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: "James F.Hranicky" <jfh@cise.ufl.edu> Cc: bsder@allcaps.org, freebsd-fs@freebsd.org Subject: Re: Non-standard root filesystems Message-ID: <3CCECFEA.88B9D86A@mindspring.com> References: <20020429153020.Q16532-100000@mail.allcaps.org> <3CCEC7D5.D22356A0@mindspring.com> <20020430130128.11428802.jfh@cise.ufl.edu>
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"James F.Hranicky" wrote:
> "Terry Lambert" <tlambert2@mindspring.com> wrote:
> > As far as software RAID is concerned: it's a bad idea, from a
> > performance perspective; I don't recommend it. Note that I'm
> > the person who did the original user space RAIDframe port to
> > FreeBSD in the mid 1990's, so I'm not just talking out my butt:
> > the amount of overhead for parity calculation and storage is
> > *considerable*, and makes RAID hardware a *much* better idea.
>
> Perhaps with RAID 5, but with 0+1 using vinum, wouldn't you
> see an increase with long enough plexes?
Only for reads. Realize that there are not load balancing
strategies available here, other than simple round-robin;
also realize that this assumes access will be non-sequential,
and that you'll actually hurt sequential access by cache
shootdown of the contents of the track cache on the physical
disk. Writes will still be screwed by having to be done twice
in order to be considered committed to stable storage.
> Granted, it's more disks, but it may be cheaper than hardware RAID
> (or not, haven't looked in a while).
>
> Plus, just a simple two disk mirror for / wouldn't be all that bad,
> considering / shouldn't get heavy write traffic, right?
IMO, / should get zero write traffic, since it should be
mounted read-only. Damn hard to install a "rootkit" on a
read-only /...
As I suggested, though... some of PHK's stuff ("GEOM", if it gets
done and gets committed) will go a ways to help the "problem".
I guess I don't totally understand the point, other than that
you can use software RAID on / on Linux and not FreeBSD? That's
not really news, and it's not really an interesting thing to do,
even if it were news.
It's actually possible to do with hoop-jumping, but it's really
not something I would recommend doing under any circumstances,
anyway.
How does Linux handle a half-plex failure detected at boot time?
It seems to me that the only way to detect this is "not booting",
which means that you're not loading any software capable of
handling the problem, anyway, and you're screwed.
Hardware RAID doesn't have this problem...
-- Terry
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