[linux-lvm] lvmcache in writeback mode

[Pim van den Berg <pim.vandenberg@nethuis.nl>]

Hi,

Since a couple of days I switched from bcache to lvmcache, running a 
Linux 3.17.7 kernel. I was surprised that it was so easy to setup 
lvmcache. :)

The system is a hypervisor and NFS-server. The LV that is used by the 
NFS-server is 1TB, 35GB SSD cache is attached (1GB metadata). One of the 
VMs runs a collectd (http://collectd.org/) server, which reads/writes a 
lot of RRD files via NFS on the LV that uses lvmcache.

My experience with bcache was that the RRD files were always in the SSD 
cache, because they were used so often, which was great! With bcache in 
writethrough mode the collectd VM had an average of 8-10% Wait-IO, 
because it had to wait until writes were written to the HDD. bcache in 
writeback mode resulted in ~1% Wait-IO on the VM. The writeback cache 
made writes very fast.

Now I switched to lvmcache. This is the output of dmsetup status:
0 2147483648 cache 8 2246/262144 128 135644/573440 366608 166900 7866816 
295290 0 127321 0 1 writeback 2 migration_threshold 2048 mq 10 
random_threshold 4 sequential_threshold 512 discard_promote_adjustment 1 
read_promote_adjustment 0 write_promote_adjustment 0

As you can see I set read_promote_adjustment and 
write_promote_adjustment to 0.

I created a collectd plugin to monitor the lvmcache usage:
https://github.com/pommi/collectd-lvmcache

Here are the results of the past 2 hours:
http://pommi.nethuis.nl/wp-content/uploads/2014/12/lvmcache-usage.png
http://pommi.nethuis.nl/wp-content/uploads/2014/12/lvmcache-stats.png

The 2nd link shows you that there are many "Write hits". You can almost 
1-on-1 map these to this graph, which shows the eth0 network packets 
(NFS traffic) on the collectd VM:
http://pommi.nethuis.nl/wp-content/uploads/2014/12/lvmcache-vm-networkpackets.png

So I think the conclusion is that the lvmcache writeback is used quite 
well for caching the collectd RRDs.

But... when I look at the CPU usage of the VM there is 8-10% Wait-IO 
(this also matches the 2 graphs mentioned above almost 1-on-1):
http://pommi.nethuis.nl/wp-content/uploads/2014/12/lvmcache-vm-load.png

This is equal to having no SSD cache at all or bcache in writethrough 
mode. I was expecting ~1% Wait-IO.

How can this be explained?

 From the stats its clear that the pattern of "Network Packets", being 
NFS traffic, matches the lvmcache "Write hits" pattern. Does lvmcache in 
writeback mode still wait for its data to be written to the HDD? Does 
"Write hits" mean something different? Is "dmsetup status" giving me 
wrong information? Or do I still have to set some lvmcache settings to 
make this work as expected?

-- 
Regards,
Pim