Reserved block count for Large Filesystem

Theodore Tso tytso at
Fri Jan 23 16:58:24 UTC 2009

On Fri, Jan 23, 2009 at 06:10:47AM -0500, Alex Fler wrote:
> On large FS like 100gb default value of "Reserved block count" takes
> 5% of usable disk, can this value be safely changed to 1% and not
> affect a performance ? Is a reservation size of 1gb enough for 100gb
> disk ? And when we have even larger filesystem like 1Tb default
> "Reserved block count" is 50GB, is it an absolutely minimum must
> have reserved number of space for disk performance, or it's just a
> legacy concept which can be adjusted?

If you set the reserved block count to zero, it won't affect
performance much except if you run for long periods of time (with lots
of file creates and deletes) while the filesystem is almost full
(i.e., say above 95%), at which point you'll be subject to
fragmentation problems.  Ext4's multi-block allocator is much more
fragmentation resistant, because it tries much harder to find
contiguous blocks, so even if you don't enable the other ext4
features, you'll see better results simply mounting an ext3 filesystem
using ext4 before the filesystem gets completely full.

If you are just using the filesystem for long-term archive, where
files aren't changing very often (i.e., a huge mp3 or video store), it
obviously won't matter.

						- Ted

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