[svlug] Distro kernels

Scott Hess scott.hess at gmail.com
Thu Mar 3 07:51:52 PST 2005


[What follows can safely be ignored by all but pedants.]

On Thu, 3 Mar 2005 06:51:27 -0800, Jeffrey Siegal <jbs at quiotix.com> wrote:
> On Mar 2, 2005, at 23:05, Scott Hess wrote:
> > Of course, Fedora wants to default you to a single giant / filesystem,
> > which nicely solves that problem.  That also solves the problem of
> > things like yum deciding to cache a couple hundred meg of packages you
> > already installed in /var.
> 
> Another way to approach this is LVM, which lets you resize (and move)
> partitions easily.  Many of these operations can even be done live
> (with the filesystem still mounted and being used).

Not quite.  You can make the filesystems hold more (or less) space in
concert with LVM, but at least with ext2/3, you can't change the
number of inodes.  Since the number of inodes determines fsck time,
there is incentive to not have an excessive number many.  On a default
install of a 120Gig drive, you get something like 14M inodes, and
fscks take a half hour or so - even though you are perhaps only
_using_ 500k inodes (most people these days have big disks to store
big files, like pictures and movies, as compared to newsfeeds or email
servers).

That's a bit of an esoteric snit, though.  ext3 means you shouldn't
need to fsck as often, though not-as-often isn't never (and shouldn't
be).  And while you can pretty safely run a stable system with /usr at
80-90% of capacity (and read-only), you generally never want 80-90% of
inodes used, because running out of inodes in a particular cylinder
group can make it impossible to allocate free space in that cylinder
group.  So there is usually room to grow.

XFS and ReiserFS appear to allocate inodes as needed, which clears
that right up.  It's been a year or two since I looked at alternatives
closely, maybe it's time to take another look.  This is another area
where my old-skool knowledge developed from reviewing 80s research is
probably marginally applicable these days, and even where it still
makes sense, the gain itself is probably marginal given smart disks
with large caches and lots of system ram.

-scott




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