Suppose an Oracle instance has to be recovered after a disaster. Do sequences get reset to the initial state, or the last saved state, or are cached values preserved?
Thank you very much. :-)
The sequnce values are stored in the SYSTEM.SEQ$ (I think) table, and a cache is maintained in memory of the next values to be used, with the size of that cache being dependent on the CACHE value for the sequence.
When the cache is exhausted the SEQ$ table is updated to a new value (in a non-consistent manner -- ie. without the user session's transacton control applying) and the next say 100 values (if CACHE=100) are read from memory.
Let's suppose that you're using a sequence with a cache size of 20. When you select a certain value from the sequence, say 1400, the SEQ$ table is updated to a value of 1420. Even if you rollback your transaction the SEQ$ still has that value until the next 20 sequence values have been used, at which time SEQ$ gets updated to 1440. If you have then just used value 1423 and an instance crash occurs, then when the system restarts the next value to be read from the sequnce will be 1440.
So, yes the integrity of the sequence will be preserved and numbers will not be "reissued". Note that the same applies to a graceful shutdown -- when you restart you will get a new value of 1440 in the above example. Sequences are not guaranteed to be gap free in practice for this reason (also because using a value and then rolling back does not restore that value to the cache).
Not that I have any experience with this, but I very much assume that a recovery to a consistent system change number state would also return the sequence to the last saved state. Anything else would be fairly useless in terms of recovery.
As for cached values, those are (can be) lost even when the instance shuts downs in an orderly manner (*): Instances cache a number of sequence values in memory (SGA) instead of going to the database every time. Unused sequence values that the instance has reserved can "disappear", leaving you with gaps in the sequence.
(*) 8i documentation mentions that this can happen with parallel instances (RAC), in which case the sequence may not even be strictly ascending (but still unique), 10g docs say that it happens in case of an instance failure.
Related
From time to time our Oracle response times decrease significally for a minute or two, without having extra load.
we were able to identify an insert statement, which produces a lot of buffer busy waits.
From the ADDM report, we got the following hint:
Consider partitioning the INDEX "IDX1" with object
ID 4711 in a manner that will evenly distribute concurrent DML across
multiple partitions.
To be honest: I am not sure what that means. I don't know what a partitioned index is. I only can Image that it means to create a Partition with a local index.
Can you help me out here?
There is a very high frequency of reading and writing to that table. no updates or deletes are used.
Thanks,
E.
I am not sure what that means.
Oracle is telling you that there is a lot of concurrent ("at the same time") activity on a very small part of your index. This happens a lot.
Consider an index column TAB1_PK on table TAB1 whose values are inserted from a sequence TAB1_S. Suppose you have 5 database sessions all inserting into TAB1 at the same time.
Because TAB1_PK is indexed, and because the sequence is generating values in numeric order, what happens is that all those sessions have to read and update the same blocks of the index at the same time.
This can cause a lot of contention -- way more than you would expect, due to the way indexes work with multi-version read consistency. I mean, in some rare situations (depending on how the transaction logic is written and the number of concurrent sessions), it can really be crippling.
The (really) old way to avoid this problem was to use a reverse key index. That way, the sequential column values did not all go to the same index blocks.
However, that is a two-edged sword. On the one hand, you get less contention because you're inserting all over the index (good). On the other hand, your rows are going all over the index, meaning you cannot cache them all. You've just turned a big logical I/O problem into a physical I/O problem!
Nowadays, we have a better solution -- a GLOBAL HASH PARTITION on the index.
With a GHP, you can specify the number of hash buckets and use that to trade-off between how much contention you need to handle vs how compact you want the index updates (for better buffer caching). The more index hash partitions you use, the better your concurrency but the worse your index block buffer caching will be.
I find a number (of global hash partitions) around 16 is pretty good.
We are using a timestamp to ensure that entries in a log table are recorded sequentially, but we have found a potential flaw. Say, for example, we have two nodes in our RAC and the node timestamps are 1000ms off. Our app server inserts two log entries within 30ms seconds of each other. The first insert is serviced by Node1 and the second by Node2. With 1000ms difference between the two nodes, the timestamp could potentially show the log entries occurring in the wrong order! (I would just use a sequence, but our sequences are cached for performance reasons... )
NTP sync doesn't help this situation because NTP has a fault tolerance of 128ms -- which leaves the door open for records to be recorded out of order when they occur more frequently than that.
I have a feeling I'm looking at this problem the wrong way. My ultimate goal is to be able to retrieve the actual sequence that log entries are recorded. It doesn't have to be by a timestamp column.
An Oracle sequence with ORDER specified is guaranteed to return numbers in order across a RAC cluster. So
create sequence my_seq
start with 1
increment by 1
order;
Now, in order to do this, that means that you're going to be doing a fair amount of inter-node communication in order to ensure that access to the sequence is serialized appropriately. That's going to make this significantly more expensive than a normal sequence. If you need to guarantee order, though, it's probably the most efficient approach that you're going to have.
Bear in mind that an attached timestamp on a row is generated at time of the insert or update, but the time that the actual change to the database takes place is when the commit happens - which, depending on the complexity of the transactions, row 1 might get inserted before row2, but gett committed after.
The only thing I am aware of in Oracle across the nodes that guarantees the order is the SCN that Oracle attaches to the transaction, and by which transactions in a RAC environment can be ordered for things like Streams replication.
1000ms? It is one sec, isn't it? IMHO it is a lot. If you really need precise time, then simply give up the idea of global time. Generate timestamps on log server and assume that each log server has it's own local time. Read something about Lamport's time, if you need some theory. But maybe the source of your problem is somewhere else. RAC synchronises time between nodes, and it would log some bigger discrepancy.
If two consecutive events are logged by two different connections, is the same thread using both connections? Or are those evens passed to background threads and then those threads write into the database? i.e. is it logged sequentially or in parallel?
I've found a gap of 18 numbers in my oracle sequence that appeared during holidays and found no traces of this crime in log files. Can I see somehow a time when sequence is updated or any other way to trace the disappearance of sequence numbers?
As the other answers/comments state, sequences are never guaranteed to be gap free. Gaps can happen in many ways.
One of the things about sequences are that they default to CACHE 20. That means that when NEXTVAL is called, the sequence in the database is actually increased by 20, the value is returned, and the next 19 values are just returned from memory (the shared pool.)
If the sequence is aged out of the shared pool, or the shared pool is flushed or the database restarted, the values "left over" in the sequence memory cache is lost, as the sequence next time picks up the value from the database, which was 20 higher than last time.
As you state it happened during holidays, I guess it to be a likely cause that your sequence used 2 values from memory, then the DBA did maintenance in the holidays that caused the shared pool to flush, and the remaining 18 values in the sequence memory cache is gone.
That is normal behaviour and the reason why sequences work fast. You can get "closer" to gap-free by NOCACHE NOORDER, but it will cost performance and never be quite gap-free anyway.
A sequence is not guaranteed to produce a gap free sequence of numbers. It is used to produce a series of (unique) numbers that is highly scalable in a multi-user environment.
If your application requires that some numeric identifier on a table is sequential without gaps then you will need another way of generating those identifiers.
The oracle sequence are NOORDER in default. please refer the oracle documentation https://docs.oracle.com/cd/B13789_01/server.101/b10759/statements_6014.htm . hence NOORDER behavior cannot be as expected. You can check whether your oracle sequence is in ORDER if not then it is some other problem.
I could see the DBA team advises to set the sequence cache to a higher value at the time of performance optimization. To increase the value from 20 to 1000 or 5000.The oracle docs, says the the cache value,
Specify how many values of the sequence the database preallocates and keeps in memory for faster access.
Somewhere in the AWR report I can see,
select SEQ_MY_SEQU_EMP_ID.nextval from dual
Can any performance improvement be seen if I increase the cache value of SEQ_MY_SEQU_EMP_ID.
My question is:
Is the sequence cache perform any significant role in performance? If so how to know what is the sufficient cache value required for a sequence.
We can get the sequence values from oracle cache before them used out. When all of them were used, oracle will allocate a new batch of values and update oracle data dictionary.
If you have 100000 records need to insert and set the cache size is 20, oracle will update data dictionary 5000 times, but only 20 times if you set 5000 as cache size.
More information maybe help you: http://support.esri.com/en/knowledgebase/techarticles/detail/20498
If you omit both CACHE and NOCACHE, then the database caches 20 sequence numbers by default. Oracle recommends using the CACHE setting to enhance performance if you are using sequences in an Oracle Real Application Clusters environment.
Using the CACHE and NOORDER options together results in the best performance for a sequence. CACHE option is used without the ORDER option, each instance caches a separate range of numbers and sequence numbers may be assigned out of order by the different instances. So more the value of CACHE less writes into dictionary but more sequence numbers might be lost. But there is no point in worrying about losing the numbers, since rollback, shutdown will definitely "lose" a number.
CACHE option causes each instance to cache its own range of numbers, thus reducing I/O to the Oracle Data Dictionary, and the NOORDER option eliminates message traffic over the interconnect to coordinate the sequential allocation of numbers across all instances of the database. NOCACHE will be SLOW...
Read this
By default in ORACLE cache in sequence contains 20 values. We can redefine it by given cache clause in sequence definition. Giving cache caluse in sequence benefitted into that when we want generate big integers then it takes lesser time than normal, otherwise there are no such drastic performance increment by declaring cache clause in sequence definition.
Have done some research and found some relevant information in this regard:
We need to check the database for sequences which are high-usage but defined with the default cache size of 20 - the performance
benefits of altering the cache size of such a sequence can be
noticeable.
Increasing the cache size of a sequence does not waste space, the
cache is still defined by just two numbers, the last used and the
high water mark; it is just that the high water mark is jumped by a
much larger value every time it is reached.
A cached sequence will return values exactly the same as a non-cached
one. However, a sequence cache is kept in the shared pool just as
other cached information is. This means it can age out of the shared
pool in the same way as a procedure if it is not accessed frequently
enough. Everything is the cache is also lost when the instance is
shut down.
Besides spending more time updating oracle data dictionary having small sequence caches can have other negative effects if you work with a Clustered Oracle installation.
In Oracle 10g RAC Grid, Services and Clustering 1st Edition by Murali Vallath it is stated that if you happen to have
an Oracle Cluster (RAC)
a non-partitioned index on a column populated with an increasing sequence value
concurrent multi instance inserts
you can incur in high contention on the rightmost index block and experience a lot of Cluster Waits (up to 90% of total insert time).
If you increase the size of the relevant sequence cache you can reduce the impact of Cluster Waits on your index.
CREATE SEQUENCE S1
START WITH 100
INCREMENT BY 10
CACHE 10000000000000000000000000000000000000000000000000000000000000000000000000
If i fire a query with such a big size even if it creates the sequence s1.
What is the max size that I can provide with it???
http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/statements_6015.htm#SQLRF01314
Quote from 11g docs ...
Specify how many values of the sequence the database preallocates and keeps in memory for faster access. This integer value can have 28 or fewer digits. The minimum value for this parameter is 2. For sequences that cycle, this value must be less than the number of values in the cycle. You cannot cache more values than will fit in a given cycle of sequence numbers. Therefore, the maximum value allowed for CACHE must be less than the value determined by the following formula:
(CEIL (MAXVALUE - MINVALUE)) / ABS (INCREMENT)
If a system failure occurs, then all cached sequence values that have not been used in committed DML statements are lost. The potential number of lost values is equal to the value of the CACHE parameter.
Determining the optimal value is a matter of determining the rate at which you will generate new values, and thus the frequency with which recursive SQL will have to be executed to update the sequence record in the data disctionanry. Typically it's higher for RAC systems to avoid contention, but then they are also generally busier as well. Performance problems relating to insufficient sequence cache are generally easy to sport through AWR/Statspack and other diagnostic tools.
Looking in the Oracle API, I don't see a maximum cache size specified (Reference).
Here are some guidelines on setting an optimal cache size.