mke2fs options for very large filesystems

[Theodore Ts'o]

On Wed, Feb 09, 2005 at 06:07:41AM -0800, Jeff Dinisco wrote:
> 
> Could you explain why filesystem performance will degrade when the last
> 5% is used?  I have several 1TB filesystems w/ only 1% reserved.  The
> man page describes this space as "reserved for the super-user".  I
> always assumed this was a buffer for root to be able to perform
> operations on a full filesystem.  It seems as though 10GB is more than
> enough.  Is this not the case?  

There are two reasons for the reserve.  One is to reserve space on the
partition containing /var and /etc for log files, etc.  The other is
to avoid the performance degredation when the last 5-10% of the disk
space is used.  (BSD actually reserves 10% by default.)  Given that
the cost of a 200 GB disks is under $100 USD the cost of reserving
10GB is approximately $5.00 USD, or approximately the cost of a large
cup of coffee at Starbucks.

Why is does the filesystem performance degrade?  Because as the last
blocks of a filesystem are consumed the probability of filesystem
fragmentation goes up dramatically?  Exactly when this happens depends
on the write patterns of the workload and the distribution of the file
sizes on the filesystem, but on average this tends to happen when the
utilization rises to 90-95%.

					- Ted