[lustre-discuss] About the data journaling mode of Lustre with ldiskfs.

[Dilger, Andreas]

On Feb 6, 2017, at 21:15, Hanggi CUI <hanggicrown at gmail.com> wrote:
> 
> Hello, I'm studying the Lustre DFS in our college.
> 
> Last month, we started to benchmark the Lustre performance, between the ordered journaling mode and the data journaling mode. We used ldiskfs as our backend FS - which mainly based on Ext3/Ext4 (added the mount option data=journal which ext3 defined). But we didn't get the desired results.
> 
> So I start to searching informations and analyzing the source code by typing printk().
> Then I found such things, like llite is the client VFS interface, LOD stripe the data, osc send data to OST, fsfilt is replaced by OSD, obdfilter named OFD which handles the read and write from OSC, etc (May not be very accurate).
> But there is no write operation functions called in ldiskfs in OST when I write a file or running fio, so it never journal the data but only metadata.
> 
> I long for someone to tell me does lustre DFS with ldiskfs have data journaling mode? and why?
> 
> Or I can get some more detailed documentations about this.

The reason that data journaling doesn't work for Lustre writes is because the OFD/OSD code has an optimized I/O path for improved performance.  Since data journaling would double the amount of data being written to disk and cut write performance by 50%, which is not something that users want.

Is there a reason for using data=journal, or just an experiment to see how things work?

Cheers, Andreas
--
Andreas Dilger
Lustre Principal Architect
Intel Corporation