Re: External journals and NVRAM devices

[Edward Shishkin <edward@namesys.com>]

JP Howard wrote:
> 
> On Thu, 31 Oct 2002 23:29:57 -0700, "Andreas Dilger"
> <adilger@clusterfs.com> said:
> > When people were testing this with ext3 external journals, they just
> > used a RAMDISK for getting the performance measurements.  Obviously,
> > (I hope ;-) this is not something you can do in real life, but for
> > performance measurement it is OK.
> >
> > Most people found that the ramdisk (and presumably the NVRAM device too)
> > didn't perform much, if any, better than having a separate fast disk for
> > the journal, because you are doing sequential I/O to the journal anyways.
> <...>
> 
> Yes, I'd heard something like this. Our servers aren't going to have a
> spare drive bay, I think, so a PCI NVRAM card may turn out to be a more
> economical solution (although I haven't received quotes back from the
> vendors yet...).
> 
> If I do find a spare drive bay, how unsafe would it be to use a single
> drive, rather than RAID 1 mirroring? What does ReiserFS do if it gets an
> IO error on the journal device? 

Reiserfs will want you to do following:
#reiserfsck --no-journal-available main_device
or 
#reiserfsck --no-journal-available --rebuild-tree main_device
then specify new journal device by reiserfstune.

>Could that bring down our whole system? I
> assume that it would--in which case using NVRAM would actually save two
> drive bays, since it should be reliable enough to not need redundency.
> 
> So, how big is a ReiserFS journal when using data=journal anyways?...

8192 blocks for reiserfs with standard journal.
The size of external journal device for non-standard.

Edward.