Re: [boost] [interprocess] semantics of mmap()d files on network filesystems

From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2007-09-14 19:46:25

Sebastian Redl wrote:
> Phil Endecott wrote:
>> Does anyone have any ideas about the behaviour of memory-mapped files 
>> on network filesystems, i.e. NFS and/or Windows shares?  Is there any 
>> chance that Boost.Interprocess could use mmaped files on such 
>> filesystems, with instances of the program running on different 
>> machines sharing data?
>>   
> MSDN has this to say for CreateFileMapping:
>> Although *CreateFileMapping* works with remote files, it does not keep
>> them coherent. For example, if two computers both map a file as
>> writable, and both change the same page, each computer only sees its
>> own writes to the page. When the data gets updated on the disk, it is
>> not merged.

Any ideas when the write-back occurs?  When the file is closed / 
unmapped?  Can you call flush to force write back?  (Does interprocess 
support any of this?)

> Although I couldn't find concrete information in the man pages, some
> Google results on "mmap consistent" suggest that mmap behaves the same.

I have had a look at the NFS spec and it says a lot about what 
implementations "may" do, and it's similar.

A simpler case than the example that you quote above is one writer and 
multiple readers.  The main requirement is that when the writer's 
changes become visible to a reader they do so atomically, or at least 
in-order.  I.e. if I have a large interprocess::list which spans 
multiple blocks, and the writer inserts something, then the readers 
must see either no changed blocks or all changed blocks.  Or, if I have 
some sort of lock-free structure like a journal, and the writer appends 
data and then updates an end-of-journal pointer to make it visible, 
then the page containing the pointer must not become visible to the 
readers until after the pages containing the data.

Anyway, I think the answer is that this is either hard or impossible; 
but I thought I'd ask just in case someone has already investigated.

Regards,

Phil.