Thread: ["Stephen C. Tweedie" ] Re: O_DSYNC flag for open

Just a quick delurk to pass along this tidbit from linux-kernel on
Linux *sync() behavior, since we've been talking about it a lot...

-Doug

Hi,

On Wed, Mar 14, 2001 at 10:26:42PM -0500, Tom Vier wrote:
> fdatasync() is the same as fsync(), in linux.

No, in 2.4 fdatasync does the right thing and skips the inode flush if
only the timestamps have changed.

> until fdatasync() is
> implimented (ie, syncs the data only)

fdatasync is required to sync more than just the data: it has to sync
the inode too if any fields other than the timestamps have changed.
So, for appending to files or writing new files from scratch, fsync ==
fdatasync (because each write also changes the inode size).  Only for
updating existing files in place does fdatasync behave differently.

> #ifndef O_DSYNC
> # define O_DSYNC O_SYNC
> #endif

2.4's O_SYNC actually does a fdatasync internally.  This is also the
default behaviour of HPUX, which requires you to set a sysctl variable
if you want O_SYNC to flush timestamp changes to disk.

Cheers,
 Stephen

Doug McNaught <doug@wireboard.com> forwards:
>> 2.4's O_SYNC actually does a fdatasync internally.  This is also the
>> default behaviour of HPUX, which requires you to set a sysctl variable
>> if you want O_SYNC to flush timestamp changes to disk.

Well, that guy might know all about Linux, but he doesn't know anything
about HPUX (at least not any version I've ever run).  O_SYNC is
distinctly different from O_DSYNC around here.
        regards, tom lane

Tom Lane <tgl@sss.pgh.pa.us> writes:

> Doug McNaught <doug@wireboard.com> forwards:
> >> 2.4's O_SYNC actually does a fdatasync internally.  This is also the
> >> default behaviour of HPUX, which requires you to set a sysctl variable
> >> if you want O_SYNC to flush timestamp changes to disk.
> 
> Well, that guy might know all about Linux, but he doesn't know anything
> about HPUX (at least not any version I've ever run).  O_SYNC is
> distinctly different from O_DSYNC around here.

Y'know, I figured that might be the case.  ;)  He's a well-respected
Linux filesystem hacker, so I trust him on the Linux stuff.  

So are we still thinking about preallocating log files as a
performance hack?  It does seem that using preallocated files along
with O_DATASYNC will eliminate pretty much all metadata writes under
Linux in future...

[NOT suggesting we try to add anything to 7.1, I'm eagerly awaiting RC1]

-Doug

Doug McNaught <doug@wireboard.com> writes:
> So are we still thinking about preallocating log files as a
> performance hack?

We're not just thinking about it, we're doing it in current sources ...
        regards, tom lane

> So are we still thinking about preallocating log files as a
> performance hack?  It does seem that using preallocated files along
> with O_DATASYNC will eliminate pretty much all metadata writes under
> Linux in future...
> 
> [NOT suggesting we try to add anything to 7.1, I'm eagerly awaiting RC1]

I am pretty sure that is done.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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Pennsylvania19026
 

[ Drifting off topic ... ]

> Well, that guy might know all about Linux, but he doesn't know anything
> about HPUX (at least not any version I've ever run).  O_SYNC is
> distinctly different from O_DSYNC around here.

There is a HP_UX kernel flag 'o_sync_is_o_dsync' which will cause
O_DSYNC to be treated as O_SYNC.  It defaults to being off -- it
is/was a backward compatibility "feature" since HP-UX 9.X (which is
history now) had implemented O_SYNC as O_DSYNC.

http://docs.hp.com/cgi-bin/otsearch/getfile?id=/hpux/onlinedocs/os/KCparam.OsyncIsOdsync.html

Regards,

Giles

> There is a HP_UX kernel flag 'o_sync_is_o_dsync' which will cause
> O_DSYNC to be treated as O_SYNC.  It defaults to being off -- it

... other way around there, of course.  Trying to clarify and
adding confusion instead. :-(

> is/was a backward compatibility "feature" since HP-UX 9.X (which is
> history now) had implemented O_SYNC as O_DSYNC.

Muttering,

Giles