Performance issues with 8.0 ZFS and sendfile/lighttpd

[Ivan Voras]

gnukix at alltel.blackberry.com wrote:
> I can send in more documentation later but I am seeing severe zfs performance issues with lighttpd. Same machine using UFS will push 1gbit or more but same content and traffic load can not hit 200mbit. Ufs does around 3 megabytes/sec IO at 800mbit network but zfs pushes the disks into the ground with 50+ megabytes/sec dusk i/o. No compression no atime no checksums on zfs and still same IO levels. Ufs with soft updates and atime on. Orders of magnitude more disk IO... Like zfs isn't using cache or isn't coalescing disk reads or both. 
> 
> Has anyone else seen this or have any recommendations? Lighttpd config remains exactly the same as well FYI. Only difference is ufs vs zfs. 

AFAIK, ZFS is incompatible (currently) with some advanced VM operations 
(like mmap, and I think sendfile relies on the same mechanism as mmap), 
so that could be a cause of the slowdown. Though I'm surprised you can 
only get 200 MBit/s - that's 25 MB/s and I think that even with multiple 
memcpy-ing data around the kernel you should be able to get hundreds of 
MB/s on newer hardware (which normally really can achieve tens of 
gigabytes/s of sustained memory access).

What else can you observe from your system? Do you have exceedingly high 
sys times and load numbers? I'm also interested in what does 10 seconds 
of running 'vmstat 1' looks like on your system. Is it a bare machine or 
a virtual machine?