[illumos-Developer] MUCH better --> zil for zvol TRUNCATE (and iSCSI UNMAP) working...

[Dan McDonald]

On Wed, Mar 02, 2011 at 09:18:02AM -0800, George Wilson wrote:
> >>  Preserving this distinction in the API seems appropriate, but there's no
> >>reason to call txg_wait_synced() in the zvol implementation.
> >You mean preserving DF_WAIT_SYNC?  Or SL_WRITEBACK_CACHE_DISABLED?
> >
> >I'll take out the txg_wait_synced() call.
> 
> Sorry guys, I feel behind on this thread.
> 
> The one thing that the txg_wait_synced() provides is for callers to
> wait for all the blocks to be freed before continuing. I don't know
> if this is a requirement or not.

Your description of txg_wait_synced() behavior does match up with the
WRITEBACK_CACHE_DISABLED, but Eric points out:

> >>It's not clear to me how a consumer
> >>could recover from "lost" frees.  For the zvol implementation this is all
> >>moot, but we can start with your interpretation and adapt it as necessary

I wonder if the principal of least surprise might apply here?  That would
argue for implementing DF_WAIT_SYNC by using txg_wait_synced().

Dan