[Openmcl-devel] ccl on ext4

Robert Goldman rpgoldman at sift.info
Tue Jul 12 14:42:31 UTC 2011


On 7/12/11 Jul 12 -9:34 AM, Tim Bradshaw wrote:
> On 12 Jul 2011, at 14:10, Gary Byers wrote:
> 
>> The Wikipedia article that Ron cites notes that a delayed allocation
>> policy (as offered by ext4, ZFS, HFS+, ...) can increase the chance
>> that on-disk data will be inconsistent in the event of a power loss
>> (or that the inconsistencies will be more severe.)  I can't think of
>> a filesystem that really guarantees disk consistency after #_write;
>> there are typically all kinds of buffering and caching mechanisms
>> involved.  (If disk consistency is important, you have to call #_fsync
>> to defeat those mechanisms; FINISH-OUTPUT calls #_fsync in CCL.)
> 
> Based on some POSIX spec-reading (so only for Unixoid systems I suppose), I think that is correct: I think the two points where you know what is on disk are after successful return of fsync or close (and a very common problem is not checking that close has returned successfully).  There may be platform-specific options which make things more aggressive than that (for instance a lot of filesystems offer "direct IO" options for use with applications which want to manage their own caching and those incidentally can mean that when writes return data is (probably) on disk since there's no OS caching in the way).
> 
> To bring this back to CL, it's slightly annoying to me that FINISH-OUTPUT &c,
> as well as CLOSE seem to offer no standard way of detecting failure.  Granted
> there's often not much you can do on failures, but being able to at least say
> "something really bad happened" would be useful.  May be I am misreading the
> spec though.

Looking at the spec, I agree that it doesn't provide any portable way to
detect failure to write.  But I don't think there's anything in the spec
that would forbid an implementation from raising a species of FILE-ERROR
if the write didn't work.

Cheers,
r




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