There is a known issue with v6.8 where parity syncs can be slower than before, mostly but not limited to faster and/or very wide arrays.
Although this isn't an urgent issue by any means this thread is meant to make it easier for LT to track the problem, also to have a better idea of how many users are affected and if there is a typical server configuration where the slowdown starts to be more noticeable.
This issue results from changes made to the md driver and tunables to fix the "writes starve reads" problem with v6.7 and based on my testing there's a general slowdown fo the max possible speed for any array size, but since most servers will be disk/controller limited those shouldn't noticed any slowdown, users with very fast and/or wide arrays can notice it.
This is from my test server using only SSDs:
You can see parity check speed with v6.8 is always slower independent of array size, in fact v6.7.2 is faster with dual parity than v6.8.0 with single parity, there's one curious exception, a read check (no parity) with 8 or 12 devices where v6.8.0 is noticeably faster, all results are consistent on repeat testing (+/- 5MB/s).
Since most servers are disk/controller limited I ran another test this time always using one slower SSD with a max read speed of 220MB/s (with single parity):
So in a typical HDD array where disks are limited from 150 to 200MB/s the user would only start noticing the issue with more than 20 devices or so.
I'm limited by the hardware I can test on but to try and rule it out as much as possible, mostly the disk controllers, I also did some tests using 2 old SAS2LP controllers (also single parity):
I can only connect up to 16 devices on both controllers (parity is on an Intel onboard SATA port), but the trend seems clear, though interestingly it's faster than before with 8 devices, this can be because the tunables used with v6.7.2 are more tuned for the LSI or the new code just works better with the SAS2LP and few devices.
Now for some real world examples, I have two 30 disks servers, on the first one parity check started at 200MB/s (disk limited), now it starts at 150MB/s, on the second server, which uses older hardware and slower disks parity check started at 140MB/s (also disk limited) and slows down just a little with v6.8 to 130MB/s, speed remains unchanged for all my other servers, mostly between 10 to 14 disks.
Anyone having the same issue please post here together with a general server description, mainly CPU, board, controller and disk types used, or post the diagnostics which contains all the info we need to see the hardware used, note that if there is a slowdown it should be more evident at the start of the parity check, which should be the max speed, average speed will likely not suffer as much, so try to compare parity check start speed after a minute or so so it can stabilize, make sure nothing else is using the array, or do a quick test in maintenance mode so you can be certain nothing else is.
Also note, that anyone using default or not optimized tunables with 6.7 might even notice a performance improvement due to the new code which attempts to auto configure the tunables for best performance.