On 2000-Jun-22 15:22:12 -0500, Chris Dillon <[EMAIL PROTECTED]> wrote:
>I think it would be a very good idea to enable softupdates by default
>when a new filesystem is created.  Modify newfs to do this and use
>tunefs only if you want to _disable_ softupdates on a filesystem.  

My only concern with doing this is softupdates behaviour when disk
space is freed.  If you delete a file, it takes about 20 seconds for
the space occupied by the file to appear in the free list.  If you
have relatively little free disk space and write another file, you
can get `disk full', even though there will be sufficient free space
when the freed blocks become available.

This has bitten a number of people who have turned softupdates on for
their root filesystems - and had installworld die.

Whilst the solution to this appears obvious (if you can't allocate a
block, but there are free blocks on the to-be-commited list, wait for
free space to become available), actually implementing it is quite
difficult if you want to avoid deadlocks.  Kirk has previously
recommended that softupdates not be enabled on a filesystem unless it
has sufficient free space to absorb about 1 minute's writes.

A number of people have also claimed that softupdates interacts with
Vinum (particularly RAID-5) in undesirable ways.  I believe that at
least some of this was a misunderstanding of the way softupdates
handles write re-ordering (or at least the write semantics that
softupdates expects).


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