On Dec 1, 2006, at 4:12 PM, Sean Murphy wrote:
I have read up on soft updates and have some questions.
The way that I am understanding soft updates purpose is to allow
file systems to be mounted dirty after an unclean shutdown of the
system. This will allow fsck to run in the background to restore
the consistency of the file system which is compared against a
snapshot of the system. It also increases performance of heavily
written file systems by waiting to write the metadata of files and
directories until a more opportune time.
I have questions about this.
When is the snapshot taken, how often, do I have to do it or does a
program or kernel do it?
Snapshots are taken via mksnap_ffs; some other tools like fsck or
dump also know how to create a snapshot.
If this is a safe way to restore consistency why is it not used on /?
You could enable softupdates on /, but normally one does not as /
does not contain files which are expected to change.
If a file system is not heavily written to is it better not to use
soft updates?
Maybe. I think that softupdates is a win in almost all circumstances
from the standpoint of data consistency, short of fully syncronous
data & metadata updates.
How do I know when the background fsck is finished and if it was
successful?
Check the logfiles.
Do I have to add anything to enable the backgound fsck?
At one point, there was an option in /etc/rc.conf, but it now
defaults to being on:
% grep fsck /etc/defaults/rc.conf
fsck_y_enable="NO" # Set to YES to do fsck -y if the initial
preen fails.
background_fsck="YES" # Attempt to run fsck in the background where
possible.
background_fsck_delay="60" # Time to wait (seconds) before starting
the fsck.
When file systems are mounted dirty and our being used while the
backgound fsck is running on the file systems how does it prevent
files from being lost?
The background fsck is only capable of handling innocuous filesystem
inconsistencies, and will fail with an error code if it encounters a
more significant issue, in which case the system is obligated to
perform the traditional fsck in the foreground.
--
-Chuck
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