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SubjectRe: 2.3.x wish list?
Date
From
Lately, tytso@mit.edu announced:
>
> Date: Fri, 21 May 1999 13:06:12 +0200 (MET DST)
> From: marcel@mesa.nl (Marcel J.E. Mol)
>
> Getting back the the mke2fs -m option. Wouldn't it be better to change the
> semantics of this option to specify the amount of blocks, MB or
> something instead of percentages? 1% of an 8GB filesystem is still 80 MB.
> I see no direct reason for a reserved area so big...
>
> There are two reasons why space is reserved in an ext2 filesystem. The
> first is as a "cushion" for root to be able to write log files, etc.
> This is most important on /usr and/or /var partitions.

If it is root that is writing, it will use the reserved space anyway so
depending on the reserved size it will take longer or shorter for
root to fill the fs whiel others can't write to it.
There might be fs that will not have any root activity, so a root cushion
is not needed. If for some reason you need emergency space you can always
borrow from other fs.

> The second
> reason is that ext2's fragmentation-avoidance algorithms work best when
> there a certain amount of slack space reserved in the filesystem (i.e.,
> we want to leave some amount of choice for the block allocation
> algorithsm to choose optimal blocks to avoid fragmentation). That's why
> -m is represented as a percentage, and why 5% is used as the default.

If the reserved space is used for allocations then in the end also the
reserved space will be fragmented. In that case it all depends on the
dynamics on the fs. But sooner or later you will need to defragment in
order to improve fs performance and restore the contigious reserved
space.

I think it just depends on the use of the filesystem how much reserved
space you might need. A sort of archive fs for example might be used
for copying old files to it without ever deleting them. As the
files are created and written one after the other you need no reserved
space as a cushion and fragmentation will not be an issue. On the
other hand an 'news' filesystem is much more dynamic and might very
well benifit from a big reserved space...

It is not that I question the use of reserved space. I just want to have
finer control over it. In the end it is just a tuning parameter. The
default can still be 5%. Allowing fraction percentages might also do
the job...

-Marcel
--
======-------- Marcel J.E. Mol MESA Consulting B.V.
=======--------- ph. 015-3101310/06-54724868 P.O. Box 112
=======--------- marcel@mesa.nl 2630 AC Nootdorp
__==== www.mesa.nl ---____U_n_i_x______I_n_t_e_r_n_e_t____ The Netherlands ____

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