Date: 26 Jan 2002 12:45:05 -0800 From: swear@blarg.net (Gary W. Swearingen) To: Brad Knowles <brad.knowles@skynet.be> Cc: "Matthew D. Fuller" <fullermd@over-yonder.net>, Mike Meyer <mwm-dated-1012390758.50933b@mired.org>, chip <chip@wiegand.org>, freebsd-chat@FreeBSD.ORG Subject: Re: Bad disk partitioning policies (was: "Re: FreeBSD Intaller (was "Re: ... RedHat ...")") Message-ID: <gc1ygc7sfi.ygc@localhost.localdomain> In-Reply-To: <p05101203b8788a930767@[10.0.1.14]> References: <20020123124025.A60889@HAL9000.wox.org> <3C4F5BEE.294FDCF5@mindspring.com> <20020123223104.SM01952@there> <p0510122eb875d9456cf4@[10.0.1.3]> <15440.35155.637495.417404@guru.mired.org> <p0510123fb876493753e0@[10.0.1.3]> <15440.53202.747536.126815@guru.mired.org> <p05101242b876db6cd5d7@[10.0.1.3]> <15441.17382.77737.291074@guru.mired.org> <p05101245b8771d04e19b@[10.0.1.3]> <20020125212742.C75216@over-yonder.net> <p05101203b8788a930767@[10.0.1.14]>
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Brad Knowles <brad.knowles@skynet.be> writes:
> At 9:27 PM -0600 2002/01/25, Matthew D. Fuller wrote:
>
> > Size doesn't matter; percentage does. I've heard somewhere
> > (from Terry, I think) that 15% is the 'optimal' setting for this, and
> > 10% was a compromise that wasn't too far below optimal, but gave that 5%
> > of extra available space. 8% is the current default in newfs(8).
>
> I disagree. Size does matter. The fragmentation-avoidance
> algorithms should still work at the sector/block/cylinder level, but the
> total disk space available is now many, many, many, many orders of
> magnitude larger than when these algorithms were first created.
I'd be good to have this documented after some more experts express a
common opinion on whether absolute or relative size of the reserve
matters and how they'd choose the numbers. I'd hope they'd speak of
partition size instead of disk size. And whether the value should
have any dependence on tunefs's -o value.
I suspect that the answer is "absolute", except for the effect big
partitions have on the willingness of the SA to reduce risks by
increasing their safety margins, at the cost of cheap disk space.
One problem is that (according to tunefs(8) man page) if one uses 5% or
less, the layout algorithm optimizes for defraging and slows down writes
"greatly". I wonder if that algorithm is obsolete. (But one can force
it to optimize for time with tunefs.)
The tunefs(8) man page leaves me wondering, when it says
This value can be set to zero, however up to a factor of three in
throughput will be lost over the performance obtained at a 10%
threshold.
whether that's true even when the filesystem is far from full or only
when comparing, say, two fileystems with 0-10% free space (and, I
suspect, only a factor of three near 0%).
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