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[hafner at almaden.ibm.com]

* From the T10 Reflector (t10 at t10.org), posted by:
* hafner at almaden.ibm.com
*


Folks,
There was an interesting discussion at the last T10 meeting on OSDs
(osd-r01).  I presented some suggestions in 00-262r0, and a reply
(00-295r0) was supplied by Dave Anderson (Seagate).  In many cases, we
agreed on many things.   A few things I raised questions about because I
wasn't clear on the issues and requirements.  A few things we disagree on
mostly in terms of implementation. But....

One issue that I want to open for discussion here is the CREATE.  I didn't
see how a CREATE could include write data (DataOut) and still get back an
ObjectID in DataIn, so I suggested separating the two operations.
Dave remarked that "this seemed wasteful in an environment where there are
a lot of small file creates" (from 00-295r0, last page, item 6).

My response is:
1) if or until there is bidirectional data on a single SCSI command, I
don't see an immediate and good alternative but...

2) one could CREATE+WRITE with no returned ObjectID, and then follow that
with a second command to request a report on the created object's ID
(that's still two commands though there is better atomicity of
CREATE+WRITE)

3) one could CREATE+WRITEw/suggested ObjectID and then the OSD can return
Status GOOD if the ObjectID was acceptable (to the OSD) and some other
status (CHECK CONDITION and include in the additional sense data the actual
ObjectID that was assigned by the OSD). This is a hack (in my opinion) but
workable, though it does distribute "namespace" responsibilities in a
different way.

4) The filesystem that expects to open lots of small files could issue a
number of CREATE commands and cache the ObjectIDs for when a real file
needs to be created.  This modifies the filesystem behavior, but is not
unreasonable.

5) We can mitigate the latency and overhead of many CREATES (as in (4)) by
having a CREATE MULTIPLE (create 'n' objects) which would return a list of
ObjectIDs.  Interesting error scenarios arise in this case, however.

Anybody else got thoughts on this?

Anybody want to bite off changing SAM to allow for bi-directional data?

Jim Hafner


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