Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> On Friday June 1, dgc@sgi.com wrote:
>   
>> On Thu, May 31, 2007 at 02:31:21PM -0400, Phillip Susi wrote:
>>     
>>> David Chinner wrote:
>>>       
>>>> That sounds like a good idea - we can leave the existing
>>>> WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED
>>>> behaviour that only guarantees ordering. The filesystem can then
>>>> choose which to use where appropriate....
>>>>         
>>> So what if you want a synchronous write, but DON'T care about the order? 
>>>       
>> submit_bio(WRITE_SYNC, bio);
>>
>> Already there, already used by XFS, JFS and direct I/O.
>>     
>
> Are you sure?
>
> You seem to be saying that WRITE_SYNC causes the write to be safe on
> media before the request returns.  That isn't my understanding.
> I think (from comments near the definition and a quick grep through
> the code) that WRITE_SYNC expedites the delivery of the request
> through the elevator, but doesn't do anything special about getting it
> onto the media.

My impression is that the sync will return when the i/o has been 
delivered to the device, and will get special treatment by the elevator 
code (I looked quickly, more is needed). I'm sore someone will tell me 
if I misread this. ;-)

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979