I infer from your response that soft updates possess:
1. increased overhead over default FFS settings.
2. increased implementation complexity over default FFS settings.
Also, I infer that journaling and soft updates provide equivalent data
safety
guarantees "in theory." Do they provide equivalent guarantees in
practice?
Thank you,
Currell
------ Original Message ------
From: "Alexandre Ratchov" <a...@caoua.org>
To: currellbe...@gmail.com
Cc: misc@openbsd.org
Sent: 1/19/2015 4:44:59 AM
Subject: Re: What are the disadvantages of soft updates?
On Mon, Jan 19, 2015 at 03:59:34AM +0000, currellbe...@gmail.com wrote:
Hello,
The FAQ[1] states that soft updates result in "a large performance
increase
in disk writing performance," and links to a resource[2] which claims
that
soft updates, in addition to being a performance enhancement, "can
also
maintain better disk consistency." Resource 2 links to several
academic
papers[3][4], which while they are a bit above my level, contain
discussions
of how soft updates can increase performance and speed recovery on
crash.
My question is: what are the downsides of soft updates?
- softdep consumes more cpu in kernel mode, which hurts interactive
programms on very slow machines. It has the reputation of
consuming more memory.
- the softdep code is more complex (likely to have more bugs).
Also, does journaling provide a better data-safety guarantee?
They are not the same. On OpenBSD, softdep makes cerain operations
much faster while ensuring that upon power loss, all
inconsistencies can be automatically fixed by fsck on next boot.
Journaling would write data twice (first in the journal, then in
the filesystem) and would allow last operations to be replayed on
next boot, so no need to run fsck, which in turn makes system boot
fast after a power loss.
In theory, from data safety point of view they are equivalent.
