Re: Proposal: B_ARRIER (addresses wapbl performance?)

[Jason Thorpe <thorpej%shagadelic.org@localhost>]

On Oct 30, 2008, at 5:08 PM, David Young wrote:

> Does FUA necessarily turn off the cache for that operation, or does it
> tell the SCSI disk to wait to indicate command completion until after
> the write is flushed from cache to stable storage?  It seems that  
> letting
> the disk flush a write on its own schedule is desirable, unless the  
> host
> has a backlog of disk ops that wait for the FUA write to complete.


FUA == Force Unit Access

The bit is defined to tell the disk that:

(read) The data should come directly from stable storage, not from any  
previously cached copy

(write) The command should not complete until the data is committed to  
stable storage (I think drives are allowed to stash a copy in their  
cache, as well, if they desire, but I'm not 100% sure)

-- thorpej