Messages in this thread |  | | | From | (Alan Cox) | | Subject | Re: scary ext2 filesystem question | | Date | Sat, 26 Dec 1998 02:58:43 +0000 (GMT) |
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> I was reading a book on filesystem design, and came across the > following scary quote:
Throw the book away, its author is incompetent to make that statement. You must therefore assume anythign else the author wrote is probably incorrect and unsuitable for learning form
> ensure that the file system is consistent, the lack of ordering on > operations can lead to confused applications or, even worse, crashing > applications because of the inconsistencies in the order of > modifications to the file system."
And he fails to understand that writing metadata first is provably the same problem
> _Practical File System Design_, Dominic Giampaolo, p. 36
Thanks. Another book never to buy
> small, and why it is nonzero, and what, if anything, an application > programmer seeking robustness could do to work around it.
Thats a different sensible question. With all Unix OS's there is a writeback cache. Thats almost always the right thing (tm). If you need to update a specific file in a safe manner and be sure the update completes perfectly you can open a file with the O_SYNC flag. This tells the unix OS that all writes must be synchronous, and they are thus completed that way.
This is still of course no guarantee. A power failure can occur during the writing of a set of synchronously updated disk blocks and then you may still lose. In fact there isn't a way to win the game only narrow the odds, evwen if your file system is uncached.
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