On 03/11/2011 08:33 PM, Indan Zupancic wrote:
On Sat, March 12, 2011 01:40, Ric Wheeler wrote:
On 03/11/2011 06:45 PM, Indan Zupancic wrote:
On Fri, March 11, 2011 12:55, Arnd Bergmann wrote:
On Friday 11 March 2011, Indan Zupancic wrote:
http://marc.info/?l=linux-fsdevel&m=127970513829285&w=2
The patch there seems much more reasonable than introducing a whole
new systemcall just for 20 lines of kernel code. New system calls are
added too easily nowadays.
The only problem with adding new system calls is that we are stuck
with the interface until the end of time, so we must be sure not
to get it wrong. The same thing is true for any other interface
such as ioctl or extensions to existing system calls. People usually
get away with adding new ioctls more easily because it is less
obvious when they are added.
Agreed.
I'm not sure this feature is important enough to add. I can't really
think of a regular use case where this would be useful, generally
it's transparent on which mount files are. Add symlinks, and you
give users a lot of rope. Any user has to make sure that all the
files they want to sync are on the same file system.
About the arguments against sync(2):
- On machines with many mounts, it is not at all uncommon for some of
them to hang (e.g. unresponsive NFS server). sync(2) will get stuck on
those and may never get to the one you do care about (e.g., /).
It would be better to fix NFS, or mount it with the fsc option (assuming
a sync will write to the local cache instead of hanging forever then).
- Some applications write lots of data to the file system and then
want to make sure it is flushed to disk. Calling fsync(2) on each
file introduces unnecessary ordering constraints that result in a large
amount of sub-optimal writeback/flush/commit behavior by the file
system.
You can use sync_file_range() on those files to schedule the writes
and then do the fsync(2) as usual (both on files and dirs).
If there still is a good reason to implement this, please don't add it
as a new system call, but add it to sync_file_range(), as that seems
the best place for odd file synchronisation operations.
Greetings,
Indan
--
Hi Indan,
I think that you missed the point of the extension.
Ric
The point is clear, it's to synchronize a specific file system instead
of all of them.
But actually doing that from a program is harder than it looks, because
programs work with files, not file systems. To make this feature useful
the program needs meta information it can't easily get. That was my first
point.
The rest was just replying to the motivations given to add the feature.
If those motivations miss the point of the extension, don't blame me.
If you want to add a new one, "I'd like sync /mounpoint to work", then
do so, but please tell what practical use that has, if you're not going
to unmount the thing soon after anyway.
Other than this questionable use case, is there any other one that would
justify adding this extension?
Greetings,
Indan
Sage was pretty clear in stating the motivation which is the use case you think
is questionable. Probably not interesting for consumer devices, but definitely
extremely interesting in large servers with multiple file systems.
In fact, we do it today as mentioned earlier in the thread - this simply exports
that useful capability in a clean way.
Ric
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