linux-ext4 - Re: [PATCH] mke2fs reserved_ratio default value is nonsensical

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Date:	Tue, 29 Mar 2011 12:57:11 -0400
From:	Oren Elrad <elrad@...ndeis.edu>
To:	unlisted-recipients:; (no To-header on input)
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mke2fs reserved_ratio default value is nonsensical

On Tue, Mar 29, 2011 at 11:26 AM, Eric Sandeen <sandeen@...hat.com> wrote:
>
> On 3/29/11 9:05 AM, Theodore Tso wrote:
> >
> > On Mar 28, 2011, at 2:06 PM, Eric Sandeen wrote:
> >>
> >> No other fs that I know of enforces this "don't fill the fs to
> >> capacity" common sense programatically, though.
> >
> > Actually, we (ext2) copied this from the BSD Fast File System (FFS)
> > which used a default MINFREE of 10%.   For ext2 we decided to bring
> > it down to 5%.   FreeBSD currently uses 8% as their default free
> > ratio.
>
> Clearly I don't know enough filesystems, I guess ;)
>
> Should have said "linux filesystem" perhaps.
>
> > The decrease does seem to be relative to the percentage of free
> > space, from empirical experience, although no one I know of has done
> > a formal analysis of the slowdown.   A lot depends on your workload,
> > how much memory pressure you place on your system, etc.  I've
> > actually started seeing slowdowns starting as early as 80% full when
> > you're trying to allocate large chunks (1M to 8M) at a time, although
> > this isn't something where I've gathered hard data; just what I've
> > noticed from looking at different systems and their performance
> > characteristics.
> >
> > Fortunately disks are cheap, and lots of people end up buying far
> > more disk space than they need, and so they naturally keep their file
> > systems well under 75-80% full.
> >
> > If someone wants to add some tuning parameters to mke2fs.conf, so
> > they can set their own personal default free ratios, or even
> > min_reserved_blocks and max_reserved_blocks settings, that's probably
> > a reasonable patch to e2fsprogs that I'd be willing to accept.
>
> Hm I thought I had sent that, but it was only for the other two
> semi-controversial behaviors.  :)
>
> I agree, it seems like at least a decent first step to make it
> more site/admin-configurable.
>
> -Eric
>

Thanks all for the informative replies.

I never meant to suggest that 5% is a bad default as a general matter
(but I dropped the ball communicating that); I agree entirely that
non-root users should not be able to degrade performance by filling up
the disk and that 5% is a very good default choice.

My only (admittedly unsupported) claim was that past some **fixed**
value there is little gain from reserving more space independently of
the size of the volume. When I get a chance, I will try to design a
benchmark to test that claim (having a few spare 12TB volumes helps)
but I fear the results will depend heavily on the usage pattern.

Oren Elrad
Dept. of Physics
Brandeis University
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