Date: Wed, 22 Dec 1999 19:25:39 -0600 (CST) From: Ryan Thompson <freebsd@sasknow.com> To: freebsd-hackers@freebsd.org Subject: Re: Default minfree performance restrictions? Message-ID: <Pine.BSF.4.10.9912221919590.49628-100000@sasknow.com> In-Reply-To: <Pine.BSF.4.10.9912201504420.30072-100000@sasknow.com>
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Hmm... Since I didn't get a single reply to this one, am I to assume that
you all think I'm nuts, and should be publicly flogged with a copy of
Greg's book for even thinking such foolish thoughts? :-)
Or... Maybe most of the more knowledgeable individuals who follow this
list are just gone for the holidays.
Seriously, though, folks, does my idea make any sense at all?
Thanks,
- Ryan
On Mon, 20 Dec 1999, Ryan Thompson wrote:
> Hello all,
>
> After creating some larger slices than I'm used to, I finally felt the
> full force of a default 8% minfree setting. So, I went to tunefs(8) to
> try and put a damper on the multiple gigabytes that aren't being made
> available to users. However, I was a bit disappointed to note that
> setting minfree at or below 5% (using integer values!) would result in
> SPACE optimization.
>
> So, on my 3.4-STABLE system, I did some hunting around.
>
> In /usr/src/sys/ufs/ffs/fs.h, I see MINFREE defaults to 8%, and default
> time optimization, like tunefs says. Nothing fancy, there.
>
> In ./ffs_alloc.c, however, I found out how the SPACE/TIME optimization is
> determined. In particular, in ffs_realloccg(), I find (from yesterday's
> -STABLE), the following snippet:
>
> /*
> * Allocate a new disk location.
> */
> if (bpref >= fs->fs_size)
> bpref = 0;
> switch ((int)fs->fs_optim) {
> case FS_OPTSPACE:
> /*
> * Allocate an exact sized fragment. Although this makes
> * best use of space, we will waste time relocating it if
> * the file continues to grow. If the fragmentation is
> * less than half of the minimum free reserve, we choose
> * to begin optimizing for time.
> */
> request = nsize;
> if (fs->fs_minfree <= 5 || /* !!! */
> fs->fs_cstotal.cs_nffree >
> fs->fs_dsize * fs->fs_minfree / (2 * 100))
> break;
> log(LOG_NOTICE, "%s: optimization changed from SPACE to TIME\n",
> fs->fs_fsmnt);
> fs->fs_optim = FS_OPTTIME;
> break;
>
> Questions:
>
> - Can the line I've marked /* !!! */ have the minimum value of 5 safely
> reduced? Eliminated? (safely = if/when a filesystem fills up, could writes
> potentially corrupt the fs?) On small partitions with many inodes, perhaps
> 5% is appropriate, but in cases like mine, where I have filesystems in
> excess of 20GB with < 0.1% fragmentation, > 5% minfree is frankly too much
> to give away.
>
> - Would it make sense to externalize this option into a header file,
> kernel config option, or perhaps tunefs itself? I'm guessing the latter
> would require modifications to our UFS implementation to allow for the
> extra parameter for each filesystem... And would definitely qualify as an
> "invasive" change. Food for thought, though :-)
>
> Any insights?
>
> I suppose I could just go ahead and try it, but, before I end up doing a
> reinstall (cd /usr/src && make blowupworld), I thought it better to ask a
> more experienced following of users :-)
>
> --
> Ryan Thompson <ryan@sasknow.com>
> 50% Owner, Technical and Accounts
> Phone: +1 (306) 664-1161
>
> SaskNow Technologies http://www.sasknow.com
> #106-380 3120 8th St E Saskatoon, SK S7H 0W2
>
>
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