This mail is to figure out the behavior of write to same file from two
different fds. As Ryan quotes in one of comments,
I think it’s not safe. in this case:
1. P1 write to F1 use FD1
2. after P1 write finish, P2 write to the same place use FD2
since they are not conflict with each other now, the order the 2 writes send to
underlying fs is not determined. so the final data may be P1’s or P2’s.
this semantics is not the same with linux buffer io. linux buffer io will make
the second write cover the first one, this is to say the final data is P2’s.
you can see it from linux NFS (as we are all network filesystem)
fs/nfs/file.c:nfs_write_begin(), nfs will flush ‘incompatible’ request first
before another write begin. the way 2 request is determine to be ‘incompatible’
is that they are from 2 different open fds.
I think write-behind behaviour should keep the same with linux page cache.
However, my understanding is that filesystems need not maintain the relative
order of writes (as it received from vfs/kernel) on two different fds. Also, if
we have to maintain the order it might come with increased latency. The
increased latency can be because of having "newer" writes to wait on "older"
ones. This wait can fill up write-behind buffer and can eventually result in a
full write-behind cache and hence not able to "write-back" newer writes.
* What does POSIX say about it?
* How do other filesystems behave in this scenario?
Also, the current write-behind implementation has the concept of "generation
numbers". To quote from comment:
uint64_t gen; /* Liability generation number. Represents
the current 'state' of liability. Every
new addition to the liability list bumps
the generation number.
a newly arrived request is only required
to perform causal checks against the entries
in the liability list which were present
at the time of its addition. the generation
number at the time of its addition is stored
in the request and used during checks.
the liability list can grow while the request
waits in the todo list waiting for its
dependent operations to complete. however
it is not of the request's concern to depend
itself on those new entries which arrived
after it arrived (i.e, those that have a
liability generation higher than itself)
So, if a single thread is doing writes on two different fds, generation numbers
are sufficient to enforce the relative ordering. If writes are from two
different threads/processes, I think write-behind is not obligated to maintain
their order. Comments?
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