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

  • From: Hua Cui <allantreycn@xxxxxxxxxxxx>
  • To: free <oracle-l@xxxxxxxxxxxxx>, jonathan@xxxxxxxxxxxxxxxxxx
  • Date: Fri, 8 Jul 2011 10:39:39 +0800 (CST)

I think there are two reasons:
1. Oracle don't want to touch the data blocks of the truncated table, because 
it is a truncate operation, not delete — that's the main reason why truncate is 
much faster than delete.
2. Oracle don't want to reconstruct the dirty blocks of the truncated table if 
the database is crash after the truncate operation — it is not necessary.

Best Regards 
Hua Cui (dbsnake)
Mobile: +86-13910122046
Oracle Recovery Last Chance:

--- 11年7月8日,周五, Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx> 写道:

发件人: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>
主题: Re: What is the purpose of segment level checkpoint before DROP/TRUNCATE of 
a table?
收件人: "free" <oracle-l@xxxxxxxxxxxxx>
日期: 2011年7月8日,周五,上午3:07

Notes in line


Jonathan Lewis

----- Original Message ----- From: "Saibabu Devabhaktuni" <saibabu_d@xxxxxxxxx>
To: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>; "free" 
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.

> 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 
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 
   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.


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