BTW I have not seen filesystem implementations doing write ordering for direct IO unless a request merge is possible (different user credentials), since you have mentioned you are from filesystem background, do you know any such implimentation?

Rajat
From: Rajat Sharma
Sent: 03-08-2013 09:25
To: neha naik; Greg Freemyer
Cc: kernelnewbies
Subject: RE: Conceptual questions about device driver

If filesystem has to guarantee write ordering, it has to serialize write request. So, only when 'A' is written to the block (completion received from block driver) filesystem can dispatch 'B' not before that. Remember that there could be failures too. So if 'A' is failed, request for 'B' is pending, would a filesystem fail that too? I guess not. Usually page cache absorbs such overwrites first than issues a single merged IO to block device, but what about direct IO? If applications need consistency guarantee against such failures, it should serialize writes. So to summarize, write ordering effort from each layers
Block driver: none
Filesystem: best effort
Application: full

Rajat
From: neha naik
Sent: 03-08-2013 02:40
To: Greg Freemyer
Cc: Rajat Sharma; kernelnewbies
Subject: Re: Conceptual questions about device driver

Thanks for the responses.  I have one more  question for Greg. I come from filesystem background and not device driver so i may be a bit confused
about the write order fidelity. I know that filesystems guarantee that. Looking from filesystem perspective, no write will be allowed on the same block until
the first write finishes. So, if 'B' is written after 'A' you can always guarantee that you will see 'B' at the end of the two writes.
  Now imagine not having a filesystem, and doing a write directly on the device. Do device drivers honour it. Should they? I imagine device driver as a kind of
queue. So any writes are always queued up one after the other so that it gives write order fidelity whether it wants to or not. Am i missing something here.

Regards,
Neha
 

On Fri, Aug 2, 2013 at 1:56 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
On Fri, Aug 2, 2013 at 1:32 AM, Rajat Sharma <fs.rajat@gmail.com> wrote:
> On Fri, Aug 2, 2013 at 2:25 AM, neha naik <nehanaik27@gmail.com> wrote:
>> Hi,
>>  I have some conceptual questions about device driver :
>>
>> 1. Write order fidelity should be maintained when submitting requests from
>> device driver to disk below.
>>     However, acknowledging these requests it is okay if we don't necessarily
>> maintain that order, right?
>>
>
> Yes it should not matter as long as application can rely on data being
> written is in order of submission.

But it can't ..... unless the write cache is turned off and it is
known the the cache is truly off.

There is no guarantee of write order in the block stack.  Not between
the filesystem and the driver.  Not between the driver and the drive.

There are at least 2 elevators shuffling the order of writes to
optimize performance.

Rajat, did you get confused?  Or were you trying to say something else?

Greg