WRITE_SAME support now in Illumos COMSTAR

The WRITE_SAME primitive is now available in Illumos as of this push:

13382:d84aa76f7cd2 Dan McDonald
937 WRITE_SAME support for COMSTAR
Reviewed by: Gordon Ross
Reviewed by: Richard Elling
Reviewed by: Robert Gordon
Approved by: Gordon Ross

Sumit Gupta wrote the original contribution, and after a bit of my own massaging, it's now in Illumos. Unlike the UNMAP push, this one did not have a lot of rewhacking (in large part due to its lower amount of direct interaction with ZFS).

The WRITE_SAME primitive works pretty much like its name. The iSCSI initiator passes in a WRITE_SAME primitive along with a single disk block. The iSCSI target then writes the same block over the range of logical block addresses specified in the command.

One set of experiments I did prior to integration was figuring out what size buffer to allocate for an I/O. In a perfect world, you don't want to do sbd_write() calls for every 512-byte block. On the other hand, you also don't want to force the kmem allocator to perform unholy tasks of allocation. I settled on a default of 128kbytes, which has a kmem_cache magazine backing it up (according to kmem stats). Users can experiment with this themselves by tweaking stmf_sbd's sbd_write_same_optimal_chunk variable. Every WRITE_SAME request, once it generates the data, consults this variable prior to allocating a block. Source-junkies can look here for the function in question.

Happy block-writing, folks!