Andreas Dilger wrote:
On Nov 01, 2002 16:38 +1100, Jeremy Howard wrote:
I'm looking at buying solid state drives / NVRAM drives for our servers
to hold an external ReiserFS journal.
We are using 2.4.20pre11, and Chris Mason's data logging patches.
I'm looking for any tips on how large the journal is when using
data=journal, and whether the external log patches are stable and work OK
in data=journal mode. Is there a command to show the current journal
size? Does the size vary over time? We need to ensure we buy a card with
enough memory so this is important information for us.
Is anyone currently using NVRAM for the journal? If so, how do you find
the performance of this configuration?
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.
If it is on a separate disk/controller from the filesystem you don't have
any seek or channel contention with the filesystem. Of course, using a
regular disk for the journal is MUCH cheaper than an NVRAM card, so you
probably want to test this out before you go ahead and buy the NVRAM card.
NVRAM devices are great for disks you are doing a lot of random I/O
on (maybe database indexes or something), because there is zero seek
latency, but for sequential I/O (like the journal) it really isn't
anything special.
Cheers, Andreas
--
Andreas Dilger
http://www-mddsp.enel.ucalgary.ca/People/adilger/
http://sourceforge.net/projects/ext2resize/
NVRAM devices are for fsync intensive operations.
--
Hans
