Re: somewhat OT query on journalling

[David Masover <ninja@slaphack.com>]

Toby Thain wrote:
> 
> On 19-Jul-06, at 11:27 AM, Payal Rathod wrote:
> 
>> Hi, ...
>> And lastly don't the journalling fs give a false sense of security to
>> the user, saying that the data is written to disk when in reality only an
>> entry is made in journal and data is still not committed to disk.
> 
> This last one is easy to answer: No. Regardless of the filesystem you're 
> using, there is no guarantee your data hits the disk until you fsync(). 
> Journalling filesystems don't change this. (And even after that, it 
> depends on the device doing the right thing :)

Interestingly, most modern IDE hard drives can not turn off write 
caching.  But, I'm guessing they have a big enough capacitor to flush 
that if you lose power.

I often turn off fsync, because it gets too abused.  There was a bug in 
Evolution where, when dragging columns, every time the display refreshed 
(as you were dragging), it would flush and fsync.  Now tell me, do I 
really need to be absolutely sure that, when recovering from a crash, my 
Evolution column widths are EXACTLY where they were while I was dragging 
the columns?

My philosophy is, unless you have a UPS device, loss of power will 
always result in lost data.  Crashes will also more often than not 
result in lost data.  So do frequent backups and have a 
managed/monitored UPS, so that when you lose power, your system flushes 
everything to disk and shuts down.