ZFS, at least on Solaris, has issue with many multiple readers of the same file, blocking after ~31 simultaneous readers (even when there are NO writers). Ran into this with a third party library which reads a large TTF to produce business PDF documents. The hundreds of reporting processes all slowed to a crawl when accessing the 20Mb Chinese TTF for reporting because ZFS was blocking.

I can't change the code since it is third party. The only way I saw to easily fix it was on system startup to copy the fonts under a new subdir in /tmp (so in tmpfs, ie RAM, no ZFS at all there ) and then softlink the dir the product was expecting to the new dir off of /tmp, eliminating the ZFS high-volume multiple-reader bottleneck.

Never had this problem with the latest EXT filesystems on my volume groups on my Linux VMs with the same 3rd party library and same volume of throughput.