[Discuss] Issuing the 'sync' command more than once (and a tangent on how not to run a high-tech company)

[mbr at arlsoft.com (MBR)]

On 6/19/2012 3:11 PM, Derek Martin wrote:
> On Sat, Jun 16, 2012 at 12:59:44PM -0400, MBR wrote:
>> On 6/12/2012 11:22 PM, Jack Coats wrote:
>>> In old SunOS days, we could issue the 'sync' command, twice, to ensure
>>> all system
>>> buffers had been written to disk.  You could experiment to see if
>>> issuing it occasionally
>>> in your script helps.  Or issue it outside the script, even in a chron
>>> might help.
>> Actually, calling 'sync' multiple times from a script really won't
>> help.  To the best of my knowledge, no Unix kernel has ever
>> contained code that counts the number of times sync() (the system
>> call that the 'sync' command issues) has been called.
> The reason I was taught to do this differs from what you put forth,
> and regardless it's certainly true that no modern Unix should ever
> require a user to run sync manually, except possibly in very rare
> circumstances.
>
> I don't claim to know the veracity of this, but I was taught (by a
> college professor who taught Unix system adminsistration as a course,
> for whatever that's worth) that the reason to sync twice (not three
> times) is that, as you say, the first call to sync schedules the
> kernel to sync the buffers, but does not necessarily complete before
> the system call returns; however (as I was told) a SUBSEQUENT call to
> the sync() system call would block until any previously scheduled sync
> had completed.  Thus, the completion of the SECOND sync command
> guarantees that the FIRST sync completed flushing the buffers to disk.
>
> Now, I certainly have not spent the time to look at the code to any
> antiquated Unix kernels to confirm whether this was ever actually true,
> anywhere.  And I don't intend to.  But it's at least plausible that it
> was true at one point in some popular Unix.  As you yourself said, for
> quite a long while now on Linux (since August of 1995), sync() actually
> does wait until the buffers are flushed.  But even that is mostly
> irrelevant as the kernel forces the buffers to be flushed periodically
> and flushes them prior to system shutdown (assuming it can, of
> course).
>
And even that won't guarantee that it's safe to power the disks down 
until you know that the data has been copied by the drive from it's 
cache to its platters.