zfs discuss - Sep 2010 - ZFS COW and simultaneous read & write of files

Hi all, I have a ZFS question related to COW and scope.

If user A is reading a file while user B is writing to the same file,
when do the changes introduced by user B become visible to everyone?
Is there a block level scope, or file level, or something else?

Thanks!

On 09/22/10 11:22, Moazam Raja wrote:
> Hi all, I have a ZFS question related to COW and scope.
>
> If user A is reading a file while user B is writing to the same file,
> when do the changes introduced by user B become visible to everyone?
> Is there a block level scope, or file level, or something else?
>
> Thanks!
>   
Assuming the user is using read and write against zfs files.
ZFS has reader/writer range locking within files.
If thread A is trying to read the same section that thread B is writing 
it will
block until the data is written. Note, written in this case means 
written into the zfs
cache and not to the disks. If thread A requires that changes to the 
file be stable (on disk)
before reading it can use the little known O_RSYNC flag.

Neil.

On Wed, Sep 22, 2010 at 12:30:58PM -0600, Neil Perrin
wrote:
> On 09/22/10 11:22, Moazam Raja wrote:
> >Hi all, I have a ZFS question related to COW and scope.
> >
> >If user A is reading a file while user B is writing to the same file,
> >when do the changes introduced by user B become visible to everyone?
> >Is there a block level scope, or file level, or something else?
> >
> >Thanks!
> 
> Assuming the user is using read and write against zfs files.
> ZFS has reader/writer range locking within files.
> If thread A is trying to read the same section that thread B is
> writing it will
> block until the data is written. Note, written in this case means
> written into the zfs
> cache and not to the disks. If thread A requires that changes to the
> file be stable (on disk)
> before reading it can use the little known O_RSYNC flag.
That''s assuming local access (i.e., POSIX semantics).  It''s
different if
NFS is involved (because of NFS'' close-to-open semantics).  It might be
different if SMB is involved (dunno).

Nico
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