[mythtv-users] Storage on SAN

[Damien Whyte]

Mark Hetherington wrote:

>How did you increase your NFS performance?
>  
>
Hello Mark,

  Firstly I'm at work and this is all from memory - and that's not my 
strong point. Please be forgiving if I start spouting absolute rubbish !

Basically I believe that the majority of my problems came from the fact 
that my frontend was diskless and was mounting its root partition over 
the network at boot time. Initially I was not passing any particular NFS 
parameters to the kernel at boot time, so I was getting the defaults. It 
seems that the defaults picked by the kernel at boot time and the 
defaults that are picked by the NFS demon that mounts a disk are 
different. This lead to me having different file systems mounted with 
different options because of the method they were originally mounted and 
caused me a lot of confusion.

I thought my NFS server was using NFS3 to serve the frontend's root 
partition, but in fact the kernel time mount of root over NFS defaults 
to NFS2 unless specified. Apparently one of the big changes from  NFS2 
to NFS3 was to default to sync rather than async mounts. Under NFS3 sync 
mounts seem to have better performance than under NFS2.  As I was 
getting my root mounted via NFS2 I was getting a poor performing sync 
mount (I had sync specified in my exports file). When I added a kernel 
boot time option to force NFS3 I got much better performance.

Some other things that I fiddled with included adding no_wdelay as an 
option to my exported filesystems in /etc/exports. From my understanding 
of this parameter this may not be beneficial in all circumstances - but 
it did seem to improve my writes.

I also played with my settings for rsize and wsize in the /etc/fstab 
mount options. Increasing them gave me better performance. I can't 
remember the value I settled on but I think it was 16 or 32 k. This may 
not be the right option for networks that are dropping packets - but for 
me with both machines connected to the same LAN it seemed the right way 
to go. BTW I had to add these as a parameters to my kernel's boot time 
mount too.

Another thing I also think may have been adversely affecting my 
performance was that I had the sync option set in fstab. This sync 
option in fstab is not in fact a NFS parameter, but rather a generic 
mount parameter. The NFS sync/async setting I previously referred to is 
set via the exports file. Unsetting this generic sync parameter in my 
fstab file also seemed to help.

Overall it seemed that any one of these changes shouldn't really explain 
why I was getting such bad performance. Once I had made all of these 
changes however, I saw a vast improvement. In truth some of them may be 
just quackery. It seems NFS configuration is somewhat arcane, and 
perhaps even a black art. ;-)

If I had time I would like to go back and try all of these changes in 
isolation and benchmark them, just for peace of mind.

 From the sound of it, your write performance at 5 megabytes/s is not 
too bad. I'm assuming that this is over 100 megabit  ethernet ?  I don't 
think you would be getting that kind of performance unless your NFS 
configuration was broadly correct. Like I said, when my performance was 
bad it was very very bad (200 kilobytes/s). You may be able to squeeze a 
bit more performance out of it by tuning some of these parameters 
though. If you are interested I'd be pleased to send you my appropriate 
config files from home.

All the best,
Damien.