[Linux-cluster] strange slowness of ls with 1 newly created file on gfs 1 or 2

[Christopher Barry]

On Wed, 2007-07-11 at 13:01 -0400, Wendy Cheng wrote:
> Christopher Barry wrote:
> > On Tue, 2007-07-10 at 22:23 -0400, Wendy Cheng wrote:
> >   
> >> Pavel Stano wrote:
> >>
> >>     
> >>> and then run touch on node 1:
> >>> serpico# touch /d/0/test
> >>>
> >>> and ls on node 2:
> >>> dinorscio:~# time ls /d/0/
> >>> test
> >>>
> >>>  
> >>>
> >>>       
> >> What have you expected from a cluster filesystem ? When you touch a file 
> >> on node 1, it is a "create" that requires at least 2 exclusive locks 
> >> (directory lock and the file lock itself, among many other things). On a 
> >> local filesystem such as ext3, disk activities are delayed due to 
> >> filesystem cache where "touch" writes the data into cache and "ls" reads 
> >> it from cache on the very same node - all memory operations.  On cluster 
> >> filesystem, when you do an "ls" on node 2, node 2 needs to ask node 1 to 
> >> release the locks (few ping-pong messages between two nodes and lock 
> >> managers via network), the contents inside node 1's cache need to get 
> >> synced to the shared storage. After node 2 gets the locks, it  has to 
> >> read contents from the disk.
> >>
> >> I hope the above explanation is clear.
> >>
> >>     
> >>> and last thing, i try gfs2, but same result
> >>>
> >>>
> >>>  
> >>>
> >>>       
> >> -- Wendy
> >>     
> >
> > This seems a little bit odd to me. I'm running a RH 7.3 cluster,
> > pre-redhat Sistina GFS, lock_gulm, 1GB FC shared disk, and have been
> > since ~2002.
> >
> > Here's the timing I get for the same basic test between two nodes:
> >
> > [root at sbc1 root]# cd /mnt/gfs/workspace/cbarry/
> > [root at sbc1 cbarry]# mkdir tst
> > [root at sbc1 cbarry]# cd tst
> > [root at sbc1 tst]# time touch testfile
> >
> > real    0m0.094s
> > user    0m0.000s
> > sys     0m0.000s
> > [root at sbc1 tst]# time ls -la testfile
> > -rw-r--r--    1 root     root            0 Jul 11 12:20 testfile
> >
> > real    0m0.122s
> > user    0m0.010s
> > sys     0m0.000s
> > [root at sbc1 tst]#
> >
> > Then immediately from the other node:
> >
> > [root at sbc2 root]# cd /mnt/gfs/workspace/cbarry/
> > [root at sbc2 cbarry]# time ls -la tst
> > total 12
> > drwxr-xr-x    2 root     root         3864 Jul 11 12:20 .
> > drwxr-xr-x    4 cbarry   cbarry       3864 Jul 11 12:20 ..
> > -rw-r--r--    1 root     root            0 Jul 11 12:20 testfile
> >
> > real    0m0.088s
> > user    0m0.010s
> > sys     0m0.000s
> > [root at sbc2 cbarry]#
> >
> >
> > Now, you cannot tell me 10 seconds is 'normal' for a clustered fs. That
> > just does not fly. My guess is DLM is causing problems.
> >
> >   
>  From previous post, we really can't tell since the network and disk 
> speeds are variables and unknown. However, look at your data:
> 
> local "ls" is 0.122s
> remote "ls" is 0.088s
> 
> I bet the disk flushing happened during first "ls" (and different base 
> kernels treat their dirty data flush and IO scheduling differently). I 
> can't be convinced that DLM is an issue - unless the experiment has 
> collected enough sample that has its statistical significance.
> 
> -- Wendy
> 
> 
> --
> Linux-cluster mailing list
> Linux-cluster at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-cluster

Where is all the time being spent? Certainly, it should not take 10
seconds.

Let me see if I get the series of events correct here, and you can
correct me where I'm wrong.

Node1:
touch is run, and asks (indirectly) for 2 exclusive write locks.
dlm grants the locks.
File is created into cache.
locks are released (now?)

local ls is run, and asks for read lock
dlm grants lock.
reads cache.
returns results to screen
lock is released

Node2:
remote ls is run, and asks for read lock
... what happens here?
I think your saying dlm looks at the lock request, and says I can't give
it to you, because the buffer has not been sync'd to disk yet.
Does node2 wait, and retry asking for the lock after some time period,
and do this in loop? Does the dlm on Node1 request the data be sync'd so
that the requesting Node2 can access the data faster?

If Pavel used dd to create a file, rather than touch, with a size larger
than the buffer, and then used ls on Node2, would this show far better
performance? Is the real issue the corner-case of a 0 byte file being
created?

Basically, I think you're saying that the kernel is keeping the 0 byte
touched file in cache, and GFS and/or dlm cannot help with this
situation. Is that correct?



-- 
Regards,
-C