On Wed, Oct 19, 2011 at 10:13:56AM -0400, David Magda wrote: > On Wed, October 19, 2011 08:15, Pawel Jakub Dawidek wrote: > > > Fsck can only fix known file system inconsistencies in file system > > structures. Because there is no atomicity of operations in UFS and other > > file systems it is possible that when you remove a file, your system can > > crash between removing directory entry and freeing inode or blocks. > > This is expected with UFS, that's why there is fsck to verify that no > > such thing happend. > > Slightly OT, but this non-atomic delay between meta-data updates and > writes to the disk is exploited by "soft updates" with FreeBSD's UFS: > > http://www.freebsd.org/doc/en/books/handbook/configtuning-disk.html#SOFT-UPDATES > > It may be of some interest to the file system geeks on the list.
Well, soft-updates thanks to careful ordering of operation allow to mount file system even in inconsistent state and run fsck in background, as the only inconsistencies are resource leaks - directory entry will never point at unallocated inode and an inode will never point at unallocated block, etc. This is still not atomic. With recent versions of FreeBSD, soft-updates were extended to journal those resource leaks, so background fsck is not needed anymore. -- Pawel Jakub Dawidek http://www.wheelsystems.com FreeBSD committer http://www.FreeBSD.org Am I Evil? Yes, I Am! http://yomoli.com
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