cost of tdb transactions on ext4 with barriers

[Olaf Frączyk]

Please look here, if you didn't do it already:

http://www.mjmwired.net/kernel/Documentation/block/barrier.txt

Best regards,

Olaf
On Fri, 2010-02-12 at 20:17 +1100, ronnie sahlberg wrote:
> FS guys have a lot to learn from SCSI.
> 
> In SCSI you have a flag, FUA, in the rpc header for each individual
> i/o (WRITE10) that says
> * you may just write this to cache if you think this benefits performance
> or
> * this i/o goes to the medium. period.   and if not, we will make you
> suffer, publicly, next time we t.10 people meet up.
> 
> Kind of what you might expect that fsync() would provide on non-broken systems.
> 
> 
> 
> regards
> ronnie sahlberg
> 
> 
> On Fri, Feb 12, 2010 at 8:07 PM, Olaf Fraczyk <olaf at navi.pl> wrote:
> > Hello,
> >
> > In XFS it depends if you have wirte cache enabled.
> > BTW. the XFS journals only metadata.
> >
> > So to be safe you need:
> > 1. Write cache off - the barriers are not needed
> > or
> > 2. Write cache on, and barriers on
> >
> > If you have writeback cache enabled and barriers off, then on power
> > failure you can get inconsistent filesystem.
> >
> > Most people claim, that disabling both barriers and write cache is
> > better in terms of performance than cache and barriers enabled.
> >
> > I personally always turn the wirteback cache off on my drives.
> >
> > The above was for XFS but I see no point that should make it for ext4
> > any different.
> >
> > It is safe to use writeback cache and have barriers disabled only if you
> > can assure that the write cache (both on controller and in hdd drive)
> > will survive crash or power outage.
> >
> > Best regards,
> >
> > Olaf
> > On Fri, 2010-02-12 at 13:57 +1100, tridge at samba.org wrote:
> >> Hi Ronnie,
> >>
> >>  > Do you really need barriers on ext4?
> >>
> >> It depends if you want your data to survive a machine crash.
> >>
> >> For my development box, I'm happy to risk losing a small amount of
> >> data on a machine crash. For a production box it's not such a good
> >> thing.
> >>
> >> I'm not aware of anything unique in ext4 that allows it to avoid data
> >> corruption on system crash with barriers off. Maybe Rusty knows of
> >> something?
> >>
> >> Cheers, Tridge
> >>
> > --
> > Olaf Frączyk <olaf at navi.pl>
> > NAVI
> > http://www.navi.pl
> > http://www.ntp.navi.pl
> >
> >

-- 
Olaf Frączyk <olaf at cbk.poznan.pl>