Sorry for the cryptic title. Not sure how to phrase it.
This is it in a nutshell: I'm running a 22-disk setup, 19 of those in a ZFS array, 15 of those backed by three port multipliers attached to SATA controllers driven by the sata_sil24 module.
When running full speed (SATA2, i.e. 3 Gbps), the operation is pretty quirky (simple read errors will throw an entire PMP into spasms for a long time, sometimes with pretty awful results).
Booting with kernel parameter libata.force=1.5G to force SATA controllers into "legacy" speeds completely fixes all issues with the PMPs.
Thing is, my ZFS pool is backed by a fast cache SSD on my ICH10R controller. Another SSD on this same controller holds the system.
Doing libata.force=1.5G immediately shaves about 100 MB/s off the transfer rate of my SSDs. For the root drive, that's not such a big deal, but for the ZFS cache SSD, it is. It effectively makes the entire zpool slower for sustained transfers than it would've been without the cache drive. Random access and fs tree lookups, of course still benifit.
I'm hoping, though, that there's some way to pass the .force=1.5G parameter on to just the three SATA controllers being backed by the sata_sil24 module. But listing the module options for this, no such option exists.
Is this possible? And if so, how?