On Fri, 15 Feb 2008, Uwe Thiem wrote: > Second, no journalled filesystem in the whole wide world can prevent > occurences of inconsisteny in case of a power cut. None, try as they > might.
This is correct. > If the journal change still resides in the > harddrive cache while your power cut occurs, boooom - inconsistency. But this isn't the reason. Harddrives know a "flush" command which - when properly used by the filesystem (and I guess reiserfs and ext3 use it properly) - forces the journal to be written before the actual change in the main file system occurs. Whence, no loss of consistency. [Of course, there are some harddrives which ignore the "flush", but this should be counted as faulty hardware. Of course, on broken hardware, no software can work as it should.] If the power loss occurs *during* flushing the journal (and thus the journal might contain nonsense) the filesystem might still use a checksum over the journal to detect this and thus preserves consistency (although I don't know whether any existing filesystem currently does this). The real problem is that during power cut the harddrive might be writing complete nonsense *somewhere* - this is not related with any caching, and no software can safe you from this problem (and what is even worse is that there is no way to detect it...) -- [email protected] mailing list
