On 12/02/18 17:15, J. Bruce Fields wrote:
On Mon, Feb 12, 2018 at 05:09:32PM +0000, Terry Barnaby wrote:
One thing on this, that I forgot to ask, doesn't fsync() work properly with
an NFS server side async mount then ?
No.
If a server sets "async" on an export, there is absolutely no way for a
client to guarantee that data reaches disk, or to know when it happens.
Possibly "ignore_sync", or "unsafe_sync", or something else, would be a
better name.
--b.
Well that seems like a major drop off, I always thought that fsync()
would work in this case. I don't understand why fsync() should not
operate as intended ? Sounds like this NFS async thing needs some work !
I still do not understand why NFS doesn't operate in the same way as a
standard mount on this. The use for async is only for improved
performance due to disk write latency and speed (or are there other
reasons ?)
So with a local system mount:
async: normal mode: All system calls manipulate in buffer memory disk
structure (inodes etc). Data/Metadata is flushed to disk on fsync(),
sync() and occasionally by kernel. Processes data is not actually stored
until fsync(), sync() etc.
sync: with sync option. Data/metadata is written to disk before system
calls return (all FS system calls ?).
With an NFS mount I would have thought it should be the same.
async: normal mode: All system calls manipulate in buffer memory disk
structure (inodes etc) this would normally be on the server (so multiple
clients can work with the same data) but with some options (particular
usage) maybe client side write buffering/caching could be used (ie. data
would not actually pass to server during every FS system call).
Data/Metadata is flushed to server disk on fsync(), sync() and
occasionally by kernel (If client side write caching is used flushes
across network and then flushes server buffers). Processes data is not
actually stored until fsync(), sync() etc.
sync: with client side sync option. Data/metadata is written across NFS
and to Server disk before system calls return (all FS system calls ?).
I really don't understand why the async option is implemented on the
server export although a sync option here could force sync for all
clients for that mount. What am I missing ? Is there some good reason
(rather than history) it is done this way ?
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