[Fwd: Re: cost of tdb transactions on ext4 with barriers]

[Olaf Frączyk]

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 cbk.poznan.pl>