RE: What is the purpose of segment level checkpoint before DROP/TRUNCATE of a table?
Date: Fri, 8 Jul 2011 10:07:51 -0400
Message-ID: <020001cc3d78$6e07b330$4a171990$_at_rsiz.com>
I now see your concern about the ability to checksum. But that is not guaranteed (or needed by the recovery model) and since those blocks are no longer assigned to a segment any contents are immaterial. If someone is doing checksums or binary diffs between datafiles through the same point in recovery, it might break their sanity check, but it does not compromise the integrity of the standby in any way I can grok.
I think Jonathan raises an interesting question that there is a potential bug and challenging race condition in direct writes, but how that relates to flushing dirty buffers through DBWR escapes me; wouldn't the same proposed route of marking the buffers invalid/free without DBWR writing them be effective there as well? I think the question is whether a direct write might write somewhere it no longer has a right to write, but perhaps I'm not entirely clear on the issue and I look forward to Jonathan explaining his concern more fully.
The query in flight question is interesting, but if the buffer is marked free the query in flight needing a read consistent version of the block should attempt to find the block and roll it back, just as with any other block that is no longer in cache. This should cancel the query with an error. That a truncate command might cancel queries in flight involving the truncated table was accepted from the first thought of the idea of truncate versus deletes. Once the truncate has been issued (whether complete or not), if a block clone you need is still valid when you call for it you can complete, but if you have to get a block from disk and undo it your query is toast because it will fail attempting to get the block. That is no worse than a snapshot too old, and someone with authority to truncate the table did truncate the table (which I hold is the same standard as someone with authority to have given you more undo space to avoid the snapshot too old did not do that). If since the original inception of the truncate command someone has adopted the stance that it should not potentially cancel queries in flight, that would be both (IMHO) a silly standard to try to achieve and, I think, impractical if the purpose of the truncate command is to be honored. If I recall correctly (and it was 20 years ago) this was referred to as allowing breakable parse locks.
Perhaps some lurker from Oracle can tune up my memory of the history this request and its implementation. At this moment I cannot remember whether it was retrofit to V6 or not. It might have only appeared in V7.1 or later. I'll see if I can find my Oracle VLDB notes.
And I continue to be unclear as to WHY Oracle needs to flush dirty buffers from truncated or dropped segments through DBWR other than it possibly being easier to code up. I look forward to being enlightened!
mwf
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]
On Behalf Of Saibabu Devabhaktuni
Sent: Friday, July 08, 2011 3:45 AM
To: Jonathan Lewis; free
Subject: 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/> 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.
- Let's say 50,000 dirty buffers were already written to disk by DBWR on primary.
- At time T1, truncate table command issued on primary.
- 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.
- 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.
-- http://www.freelists.org/webpage/oracle-lReceived on Fri Jul 08 2011 - 09:07:51 CDT