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. (I
excluded HEAD OF QUEUE commands from the consideration for simplicity.)
Vlad
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