Re: Sync semantics.

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 11, 2010 at 01:52:19PM +0100, Rogier Wolff wrote:
> 
> Hi, 
> 
> What should I expect from a "sync" system call?
> 
> The manual says: 
> 
>    sync() first commits inodes to buffers, and then buffers to disk.
> 
> and then goes on to state: 
> 
>    ... since  version  1.3.20 Linux does actually wait.
> 
> [for the buffers to be handed over to the drive]
> 
> So how long can I expect a "sync" call to take? 
> 
> I would expect that all buffers that are dirty at the time of the
> "sync" call are written by the time that sync returns. I'm currently
> bombarding my fileserver with some 40-60Mbytes per second of data to
> be written (*). The fileserver has 8G of memory. So max 8000 Mb of
> dirty buffers can be stored, right? The server writes an average of
> (at least) 40Mb/second to disk. According to my calculator, I will
> have to wait up to 200 seconds for the sync system call to return....
> 
> 
> # time sync
> 0.000u 0.220s 2:22:23.96 0.0%   0+0k 0+0io 2pf+0w

Depending on the kernel, sync will keep writing if you keep
dirtying. This should be mostly fixed in 2.6.36....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com