Date: Fri, 1 Dec 2006 16:25:12 -0800 From: Chuck Swiger <cswiger@mac.com> To: Sean Murphy <smurphy@calarts.edu> Cc: freebsd-questions@freebsd.org Subject: Re: Soft Updates Help Message-ID: <671FF6D7-8F30-44DF-A8ED-2456E9B80170@mac.com> In-Reply-To: <4570C4D6.5030708@calarts.edu> References: <4570C4D6.5030708@calarts.edu>
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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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