[nylug-talk] Ext3 Disk Reservation and Defaults -- WAS: extfs journalling percentage

[Chris Knadle]

> Michael Bubb wrote:
...
> > I have noticed a 5.1% 'overhead' on the
> > difference between the partition's capacity and its
> > amount usable.
> > I realize this is the journal and the blocks reserved
> > for root.
>
> Chris Knadle wrote:
> > I'm not 100% positive, but I don't think the ext3
> > journal counts in that 5% reserved for root-only usage.

...
On Wednesday 28 May 2008, Bryan J. Smith wrote:
> Fragmentation is the most common issue with filesystems filling up.
> This is especially the case with user data filesystems where file
> sizes vary greatly**.  The 5% default is the threshold where this is
> most likely to occur, although it varies on the type and size of
> files, frequency, etc...  I've actually reserved 10% on some
> filesystems myself, especially if the file sizes are quite large, or
> some operations require contiguous blocks.

   Ah.  Yeah, at boot time the normal periodic fsck of ext3, when I used it, 
usually noted about 4% "non-continuous", i.e. fragmentation, if I remember 
correctly.  I'm mostly using XFS currently, in which an fsck at boot time is 
essentially a no-op.

...
> **NOTE:  Some filesystems use Extents to manage varying file size
> commits better from the standpoint of fragmentation.  E.g., XFS does
> this.  Extents also introduce overhead, so it's not ideal when the
> file sizes do not vary, or use lots of small files.  E.g., I would
> never use XFS for /tmp, /var, etc... ;)

   Interesting.

...
> The 5% default is a proliferated "good practice," although it can be
> 2% in some cases on very large Ext3 filesystems.

   The default of 5% seems like a reasonable value as far as I can tell.

   -- Chris

-- 

Chris Knadle
Chris.Knadle at coredump.us