Reserved space

Theodore Tso tytso at
Tue Aug 28 14:28:42 UTC 2007

On Tue, Aug 28, 2007 at 03:29:46PM +0200, beevis at wrote:
> Hello list,
> I have a doubt. When creating an ext3 fs, 5% of its space is reserved to the superuser.
> I understand this should be important for /, maybe /var and /tmp.
> But is it compulsory for other fs, like, say, an external disk with data?
> Or it's just a heritage, no more needed? Could one safely reclaim this 5%?
> I understand that no other fs (jfs, xfs, reiser) reserves some space.

You can, but the performance of the filesystem will go down as you use
the last 5%, especially if the filesystem is dynamic and constantly
changing, since it will cause the files to become very badly
fragmented.  UFS historically used 10% for its anti-fragmentation
reserve.  With ext3 we decreased it to 5%.

If the filesystem is going to be essentially static after you fill it
up, sure you can reduce it down to 0%.  But if the filesystem is going
to be continuously active, you will get better performance by buying a
bigger hard drive and using a filesystem with an average utilization
of 50-80% than one which is hovering between 99-100%.  Aside from
spending 100-200 Euro's on extra memory, speading 100-200 Euro's on a
newer, bigger hard drive can be one of the easist and cheapest way to
improve the performance of your system.


					- Ted

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