On Wed, Dec 31, 2014 at 12:23:00PM +0100, Pim van den Berg wrote: > 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? I think your expectations of writeback mode are correct, but to spell it out here some pseudo code. In writeback mode: if block is on ssd write to ssd, complete bio once written increment write hit counter else write to origin and complete increment write miss counter writethrough mode: if block on ssd write to ssd, then origin, complete increment write hit counter else write to origin and complete increment write miss counter Some things that can slow down IOs: - Changing a mapping due to the promotion or demotion of a block requires and metadata commit. (Check LVM2 has put the metadata on the ssd rather than spindle). - REQ_DISCARD. This is an expensive operation. I advise people to periodically use fstrim rather than having the fs do it automatically when it deletes files. - Background writeback IO could possibly be interferring with incoming writes. eg, if a dirty block is being written back when a write to that block comes in then the write will be stalled. Looking at the code I can see we're being very agressive about writing everything back irrespective of how recently the block was hit. It would be trivial to change it to only writeback after a number of policy 'ticks'. I'll do some experiments ... - Joe _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
