[lustre-discuss] Relaxing read consistency, from other node write

[Dilger, Andreas]

You can set "lctl set_param ofd.*.sync_on_lock_cancel=never" to drop the sync when multiple clients access the data before it is written to disk. 

As you wrote, it does mean if both the client and the server crash before the data is on disk it will be lost, but that is rare and in many cases the performance is more important. 

Cheers, Andreas

> On May 9, 2016, at 01:15, Hans Henrik Happe <happe at nbi.ku.dk> wrote:
> 
> Hi,
> 
> Some users experienced that reading a log file written on another node the read of the last bytes were sometimes delayed teens of seconds. This happens when other processes are writing heavily.
> 
> It seems that the data needs to be committed to persistent storage, before the reading node can have it. That makes sense since the writing node and the server could die, taking with them all knowledge about the write. Is this a correct description?
> 
> I'm wondering if there is a way to relax this. I.e. ignore this failure scenario or treat the cache entries in writing node and server as enough redundancy?
> 
> WRT why we see these long delays I think I tracked it down to an ZFS issue (https://github.com/zfsonlinux/zfs/issues/4603), but I'm only a layman when it comes to the internals of ZFS and Lustre.
> 
> We are at 2.7.64, so we have to update to 2.8 soon. Going through the commits I couldn't find anything that relates, but that might just be my ignorance.
> 
> Cheers,
> Hans Henrik
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