Re: [RFC] relaxed barrier semantics

[Vladislav Bolkhovitin <vst@vlnb.net>]

Vladislav Bolkhovitin, on 07/30/2010 05:22 PM wrote:
> Tejun Heo, on 07/30/2010 05:07 PM wrote:
>> On 07/30/2010 02:56 PM, Vladislav Bolkhovitin wrote:
>>> 1. 1st journal write command (SIMPLE)
>>>
>>> 2. 2d  journal write command (SIMPLE)
>>>
>>> 3. 3d  journal write command (SIMPLE)
>>>
>>> 4. SYNCHRONIZE_CACHE for blocks written by those 3 commands (ORDERED)
>>>
>>> 5. Necessary amount of meta-data update commands (all SIMPLE)
>>>
>>> 6. SYNCHRONIZE_CACHE for blocks written in 5 (ORDERED)
>>>
>>> 7. Command marking the transaction committed in the journal (ORDERED)
>>>
>>> That's all. No queue draining anywhere. Plus, sending commands
>>> without internal order requirements as SIMPLE would allow the drive
>>> to better schedule execution of them among internal storage (actual
>>> disks).
>>
>> Are SIMPLE commands ordered against ORDERED commands?  Aren't ORDERED
>> ordered among themselves only?
>
> About SIMPLE commands SAM says: "The command shall not enter the enabled
> command state until all commands having a HEAD OF QUEUE task attribute
> and older commands having an ORDERED task attribute in the task set have
> completed"
>
> About ORDERED commands: "The command shall not enter the enabled command
> state until all commands having a HEAD OF QUEUE task attribute and all
> older commands in the task set have completed".
>
> In a normal language it means that ORDERED commands are ordered against
> all other commands: no SIMPLE command can be executed before ORDERED
> commands ahead of it completed and no ORDERED command can be executed
> before all SIMPLE and ORDERED commands ahead of it completed.

...and, of course, SIMPLE commands can be freely reordered against 
neighbor SIMPLE commands.

> Vlad