>On Thu, 28 Apr 2005, Bryan Henderson wrote:
>> >O_SYNC doesn't work completely on several file systems and only on the
>> >latest kernels with some of the common ones.
>>
>> Hmmm. You didn't mention such a restriction when you suggested fsync()
>> before. Does fsync() work completely on these kernels where O_SYNC
>> doesn't? Considering that a simple implementation of O_SYNC just does
the
>> equivalent of an fsync() inside every write(), that would be hard to
>> understand.
>
>Some file systems implement their fsync() function as "return 0;" so no,
>you cannot rely on it at all.
It's pretty clear Alan isn't talking about those cases. I don't think he
would have suggested fsync() to address the delayed write error problem in
a case where fsync() is "return 0;".
But let's talk about the no-op fsync() cases: fsync() is supposed to
cause data to be written to stable storage. "stable" is a relative
concept that the individual filesystem type or driver has to define for
itself. In an ordinary disk-based filesystem, we usually expect it to
mean the data has gone onto the oxide. But that's not really stable --
the disk drive could break and the data would be gone. For some, just
getting into the buffers of the disk drive is stable enough, since then
rebooting Linux wouldn't cause the data to be lost. For ramfs, the Linux
page cache is as stable as you can hope for.
So I view it as correct even if fsync() does nothing on a disk-based
filesystem because the programmer was lazy (or because the user wants to
defeat the performance-busting behavior of some paranoid application). But
when Alan speaks of a "not completely correct" version of synchronization,
which makes me think of something that doesn't implement any consistent
form of "stable," I want to hear more.
--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems
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