Re: What is the purpose of segment level checkpoint before DROP/TRUNCATE of a table?

Inline responses (lines starting with "=>").

Thanks,
Sai
http://sai-oracle.blogspot.com

Notes in line Regards Jonathan Lewis http://jonathanlewis.wordpress.com ----- Original Message ----- From: "Saibabu Devabhaktuni" <saibabu_d_at_xxxxxxxxx> To: "Jonathan Lewis" <jonathan_at_xxxxxxxxxxxxxxxxxx>; "free"<oracle-l_at_xxxxxxxxxxxxx>
Sent: Thursday, July 07, 2011 5:07 PM Subject: Re: What is the purpose of segment level checkpoint before DROP/TRUNCATE of a table?
Hi Jonathan, If we have a table with 100,000 dirty buffers on primary database. 1) Let's say 50,000 dirty buffers were already written to disk by DBWR on
primary.
2) At time T1, truncate table command issued on primary. 3) If Oracle had this feature to not write buffers as part of truncate, then at
>time T2 Oracle finished marking buffers not to write.
4) At time T3, truncate operation is fully completed on primary.
5) On the standby, configure very small buffer cache size (i.e. can only fit
10,000 buffers).
6) As the redo up to time T1 applied on the standby, only 10,000 dirty buffers can stay in the cache and rest of the blocks will be written to disk as soon as
>redo is applied. 7) Redo as of time T2, will not really mark buffers as not to write, as most of
>them are already on disk. This is when blocks on the standby is going to be
different from the primary as of same checkpoint time. So what. I've already pointed out that the standby physical files are always likely to be different from the primary files at any point in time. Clearly Oracle has to be able to deal with that problem because we can always point out that Oracle is supposed survive a global powercut because of the basic redo and recovery mechanism.

=> I guess I'm not stating it properly. If you do normal shutdown of both primary and the physical standby as of time T3, and then open both of them in read only mode, checkpoint time of all datafiles on both standby and primary should be same, but some of the blocks belong to the table truncated can be different at binary level across standby and primary, but not logically. This defies the whole premise of primary and physical standby being one and the same at binary level when both of them are brought to the same checkpoint. In other words if I do "checksum" on datafiles at OS level, that should be same on primary and standby when both of them are at same checkpoint level and no further changes are happening.


>
>Data loss scenario:
1) If there was a system or datafile level checkpoint finished on primary
between time T2 and T3.
2) If primary instance crashes between time T2 and T3, but after the above
checkpoint was completed.
3) Above checkpoint would have skipped writing buffers marked as not to write
and hence on disk image is not current. 4) When instance is starting up, crash recovery starts as of redo from the most
>recent checkpoint.
5) After the completion of crash recovery, truncate never really finished, but
the data in the dirty blocks as of time T1 is missing. 
Points to worry about - what does it mean to say: truncate operation is fully completed - what are the events, and in what sequence why do you assume that you "mark the buffers as free" (t2) before you complete the truncate (t3)
if you have a checkpoint that finishes between t2 and t3 - what does it mean to say that the checkpoint finishes in this context ? when did the checkpoint start, and does that matter ? (before t1, between t1 and t2, between t2 and t3)
where do the local writes come into it in your scenario where do the updates to the data dictionary come in your scenario How about this for a truncate sequence. a) Grab an exclusive pin on the table definition - this stops any more SQL referencing the table from being compiled
(may have to wait) b) Grab an exclusive pin on every cursor referencing the table definition - this stops any SQL referencing the table from being executed
(may have to wait)
c)    Invalidate table and index entries in dictionary cache
d)    Invalidate all cursors referencing the table
e)    Generate redo for correcting segment header blocks etc.
f)    Apply, commit, and generate local writes        *** g) Generate redo for the data dictionary to show table (and indexes) have been truncated, data_object_id changed.
h)    apply and commit
i)    mark as free all buffers for table and indexes and move to REPL-AUX have to ignore blocks on write queue that are already pinned by dbwr
j)    release cursor pins
k)    release object pin *** Point (f) needs further thought - Oracle must have a mechanism for avoiding a race condition for blocks which are subject to local writes when a checkpoint is running or you get the option for something similar to your inconsistency description: dirty block is not written to file during checkpoint because it's supposed to be a local write
local write doesn't take place (for some reason, e.g. session crashes) checkpoint completes if a recovery is required very soon afterwards (and the local write still hasn't happened) then
the block on disc is wrong the recovery process is going to start from the next redo log, and therefore not see the redo that should bring the block up to date.



=> Assuming "local writes" from step f above meaning "direct path writes". Why can't they be left to be written by DBWR instead. Also, what happens if user hit CTRL-C between step f and step h.

=> Any select queries in flight will continue  to execute and fetch the records for quite a bit of time, before getting ORA-8103 error, after truncate command is completed. These select queries may see stale data if dirty buffers are flushed out without writing to the disk.

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