Not sure about the seg_max value (even in the current code).
Looking at the qemu implementation, I suspect it should be queue length -2, which I guess is what the patched code happens to achieve (based on the definition of VTBLK_RINGSZ), but it could be more explicit.

In D18831#402109, @araujo wrote:

Hi Richard,

Do you mind share the tests you made??

Best,

In D18831#402095, @pmooney_pfmooney.com wrote:

This is effectively what we did in SmartOS/illumos to address the issue:
https://github.com/joyent/illumos-joyent/commit/192e1e6405f98e4b0a12f9488793c5dd000f3f7e

I would suggest including a comment where vbc_seg_max is set to note why that value is clamped as it is.

Added comment where vbc_seg_max is set to note why that value is clamped as it is (as recommended by pmooney_pfmooney.com).

Fix cut-and-paste error in patch