Ts'o: Delayed allocation and the zero-length file problem [LWN.net]

Ts'o: Delayed allocation and the zero-length file problem

Posted Mar 14, 2009 5:45 UTC (Sat) by Nick (guest, #15060)
In reply to: Ts'o: Delayed allocation and the zero-length file problem by forthy
Parent article: Ts'o: Delayed allocation and the zero-length file problem

> "POSIX allows this" is an anal point. Like
> "the C standard allows this". This is not
> what standard documents are about.

To everyone whining about this behaviour because "posix allows it", it is not like a C compiler that adds some new crazy optimisation that can break any threaded program that previously was working (like speculatively writing to a variable that is never touched in logical control flow). POSIX in this case *IS* basically ratifying existing practices.

fsync() is something that you have to do on many OSes and many filesystems for a long time in order to get correct semantics. Programmers that come along a decades or two later and decide to ignore basics like that because it mostly appears to work on one filesystem on one OS cannot complain that a new filesystem or OS breaks their app really don't have a high horse on which to sit.... a mini pony at best.

It's quite fair to hope for a best effort, but it is also fair to make an appropriate choice about the performance tradeoff so that well written applications don't get horribly penalised.

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Ts'o: Delayed allocation and the zero-length file problem

Posted Mar 20, 2009 13:48 UTC (Fri) by anton (guest, #25547) [Link]

[...] it is not like a C compiler that adds some new crazy optimisation that can break any threaded program that previously was working (like speculatively writing to a variable that is never touched in logical control flow)

That's actually a very good example: ext4 performs the writes in a different order than what corresponds to the operations in the process(es), resulting in an on-disk state that never was the logical state of the file system at any point in time. One difference is that file systems have been crash-vulnerable ("crazy") for a long time, so in a variation of the Stockholm syndrome a number of people now insist that that's the right thing.