When Sql Server issues a checkpoint, does it block every other operation until the checkpoint is completed?
If I understand correctly, when a checkpoint occurs, the server should write all dirty pages.
When it's complete, it will write checkpoint to the transaction log, so in case of any failure it will process only transactions from that point of time (or transactions which already started at time of checkpoint).
How does sql server prevent some non dirty page to become dirty while the checkpoint is in progress?
Does it block all writes until the checkpoint is completed?
Checkpoints do not block writes.
A checkpoint has a start and an end LSN. It guarantees that all pages on disk are at least at the start LSN of the checkpoint. It does not matter if any page is at a later LSN (because it has been written to after the checkpoint has started).
The checkpoint only guarantees a minimum LSN for all pages on disk. It does not guarantee an exact LSN.
This makes sense because you can delete all transaction log records which contain information from LSNs which are earlier than the checkpoint start LSN. That is the purpose of a checkpoint: Allow parts of the log to become inactive.
Checkpoints are not needed for data consistency and correctness. They just free log space and shorten recovery times.
when a checkpoint occurs, the server should write all dirty pages
And that's what it does. However the guarantee given by checkpoint is it writes all the pages that were dirty at the instant the checkpoint started. Any page that got dirty while the checkpoint was making progress may or may not be written, but is sure not guaranteed to be written. What this guarantee offers is an optimization that physical recovery can start REDO from the last checkpoint since everything in the log prior to it is already been applied to the data pages (does not have to be redone). Is even on Wikipedia page for ARIES:
The naive way for checkpointing involves locking the whole database to
avoid changes to the DPT and the TT during the creation of the
checkpoint. Fuzzy logging circumvents that by writing two log records.
One Fuzzy Log Starts Here record and, after preparing the checkpoint
data, the actual checkpoint. Between the two records other logrecords
can be created
usr's answer explains how this is achieved (by using a checkpoint start LSN and end LSN).
Related
I want to know the detailed failure recovery process of flink.In standalone mode, I guess some steps, such as a TaskManager failure, first detect the failure, all tasks stop processing, and then redeploy the tasks. Then download the checkpoint from HDFS, and each operator loads the state. After the loading is completed, the source continues to send data. Am I right? Does anyone know the correct and detailed recovery process?
Flink recovers from failure through checkpoints. Checkpoints can be stored locally, in S3 or HDFS. When restored, all states of different operators will be revived.
For detailed recovery process, it really depends on your backend. If you are using RocksDB.
your checkpoint can be incremental
you can use the checkpoint data as a savepoint if you do not need to change the backend. This means you can change the parallelism while reviving from the checkpoint.
We have a huge DML script, that opens up a transaction and performs a lot of changes and only then it commits.
So recently, I had triggered this scripts (through an app), and as it was taking quite an amount of time, I had killed the session, which triggered a ROLLBACK.
So the problem is that this ROLLBACK took forever and moreover it was hogging a lot of CPU (100% utilization), and as I was monitoring this session (using exec DMVs), I saw a lot of waits that are IO related (IO_COMPLETION, PAGE_IO_LATCH etc).
So my question is:
1. WHy does a rollback take some much amount of time? Is it because it needs to write every revert change to the LOG file? And the IO waits I saw could be related to IO operation against this LOG file?
2. Are there any online resources that I can find, that explains how ROLLBACK mechanism works?
Thank You
Based on another article on the DBA side of SO, ROLLBACKs are slower for at least two reasons: the original SQL is capable of being multithreaded, where the rollback is single-threaded, and two, a commit confirms work that is already complete, where the rollback not only must identify the log action to reverse, but then target the impacted row.
https://dba.stackexchange.com/questions/5233/is-rollback-a-fast-operation
This is what I have found out about why a ROLLBACK operation in SQL Server could be time-consuming and as to why it could produce a lot of IO.
Background Knowledge (Open Tran/Log mechanism):
When a lot of changes to the DB are being written as part of an open transaction, these changes modify the data pages in memory (dirty pages) and log records (into a structure called LOG BLOCKS) generated are initially written to the buffer pool (In Memory). These dirty pages are flushed to the disk either by a recurring Checkpoint operation or a lazy-write process. In accordance with the write-ahead logging mechanism of the SQL Server, before the dirty pages are flushed the LOG RECORDS describing these changes needs to be flushed to the disk as well.
Keeping this background knowledge in mind, now when a transaction is rolled back, this is almost like a recovery operation, where all the changes that are written to the disk, have to be undone. So, the heavy IO we were experiencing might have happened because of this, as there were lots of data changes that had to be undone.
Information Source: https://app.pluralsight.com/library/courses/sqlserver-logging/table-of-contents
This course has a very deep and detailed explanation of how logging recovery works in SQL Server.
In which conditions the T-logs are flushed from the log cache to log file or disk?
Does it happen after every commit or after every 3 seconds or only after checkpoint?
And in where the dirty pages are stored in the SQL server when the memory is not big enough to hold the data in the buffer pool(in temp db or in the respective databases)? and for how long the uncommitted data is preserved in SQL server and where?
You are asking two random questions
1.Transaction log buffer
2.Buffer pool
In which conditions the T-logs are flushed from the log cache to log file or disk?Does it happen after every commit or after every 3 seconds or only after checkpoint?
Consider below update statement
Update table set id=1
where id=2
First of all this modification is written to Transaction log buffer..SQLServer then writes this modification to disk, before we get successfull commit..This is called write Ahead logging and this type of commits will not be periodic or any thing..This happens per statement
And in where the dirty pages are stored in the SQL server when the memory is not big enough to hold the data in the buffer pool(in temp db or in the respective databases)? and for how long the uncommitted data is preserved in SQL server and where?
consider the same update transaction and this update needs to touch three pages..and one page is not in buffer pool..In this case,SQL reads the page from disk and places it in buffer pool and modifies it..Now this page is called dirty page...
These type of pages will be flushed to disk ,When check point occurs..check point occurs due to various conditions as mentioned in below link
https://msdn.microsoft.com/en-us/library/ms189573.aspx
Checkpoint is an internal process that writes all dirty pages (modified pages) from Buffer Cache to Physical disk, apart from this it also writes the log records from log buffer to physical file.
A checkpoint always writes out all pages that have changed (known as being marked dirty) since the last checkpoint, or since the page was read in from disk. It doesn't matter whether the transaction that changed a page has committed or not – the page is written to disk regardless. The only exception is for tempdb, where data pages are not written to disk as part of a checkpoint.
A checkpoint is only done for tempdb when the tempdb log file reaches 70% full – this is to prevent the tempdb log from growing if at all possible (note that a long-running transaction can still essentially hold the log hostage and prevent it from clearing, just like in a user database).
Conditions when the Transaction logs are flushed into the Log File:
The LOGWRITER is the process which is responsible for writing logs from Log Cache to Log file.
The conditions where the log buffer is flushed to disk includes:
A session issues a commit or a rollback command.
The log buffer becomes 1/3 full.
After every checkpoint.
Whenever Log file becomes 70% Full.
It also depends on the Target Recovery Time
Does it happen after every commit or after every 3 seconds or only
after checkpoint?
It happens after every commit and after every checkpoint.
Checkpoint occurs for a user database, all dirty pages for that database are flushed to disk (as well as other operations). This does not happen for tempdb. Tempdb is not recovered in the event of a crash, and so there is no need to force dirty tempdb pages to disk, except in the case where the lazywriter process (part of the buffer pool) has to make space for pages from other databases. When you issue a manual CHECKPOINT, all the dirty pages are flushed, but for automatic checkpoints they’re not.
Checkpoints
How long the uncommitted data is preserved in SQL server and where?
SQL Server will keep the uncommitted data in the data and log files unless and until the transaction is completed / rolled back.
I have 2 sessions, each performs the same tasks but on different tables, as follows
begin tran
update...set...
commit tran
checkpoint
Each update is a large batch. The database is under simple recovery mode. To save t-log from growing too large we issue checkpoint so that t-log truncation can happen.
My question is:
If session A committed the transaction and issued a checkpoint while session B is still in the process of updating will the checkpoint issue by session A wait on session B due to session B's active transaction?
In other words would a checkpoint have to wait for all active transaction to finish? How likely is it for the two sessions to form a deadlock?
Also if two checkpoint commands are issued at the same time what will happen?
Note that the session A updates table_A and session B updates table_B. They never update the same table at any given time.
Also I know that using insert into, rename, drop can achieve faster update. But I am limited not to do so. I just want to know about checkpoint concurrency.
Thanks,
A manual Checkpoint simply tells SQL Server to write in-memory changes to disk. It should have no effect on the size of the log.
If Session A commits and checkpoints while Session B is in a transaction on a different table, these are unrelated events - the Session A checkpoint will go ahead, as will the Session B transaction. Since a manual checkpoint simply forces a write of the in-memory data to disk at a time of the programmer's choosing rather than at a time of SQL Server's choosing, the only perceptible consequence should be slightly degraded performance.
Since checkpoints take effect at the database level, concurrent Checkpoints should have the same effect as one Checkpoint.
Checkpoints have absolutely no relation to the data in the database. They do not cause data changes or changes in visibility.
You are likely degrading performance considerably.
Also, it is unlikely that this solves your log problems because SQL Server by default checkpoints regularly anyway. Learn about the log a little more and you'll surely find a better way to address that. Or ask a question about your log problems.
I read the following statement:
SQL Server doesn’t write data immediately to disk. It is kept in a
buffer cache until this cache is full or until SQL Server issues a
checkpoint, and then the data is written out. If a power failure
occurs while the cache is still filling up, then that data is lost.
Once the power comes back, though, SQL Server would start from its
last checkpoint state, and any updates after the last checkpoint that
were logged as successful transactions will be performed from the
transaction log.
And a couple of questions arise:
What if the power failure happens after SQL Server issues a
checkpoint and before the buffer cache is actuall written to
disk? Isn't the content in buffer cache permanently missing?
The transaction log is also stored as disk file, which is no
different from the actual database file. So how could we guarantee
the integrity of log file?
So, is it true that no real transaction ever exists? It's only a matter of probability.
The statement is correct in that data can be written to cache, but misses the vital point that SQL Server uses a technique called Write Ahead Logging (WAL). The writes to the log are not cached, and a transaction is only considered complete once the transaction records have been written to the log.
http://msdn.microsoft.com/en-us/library/ms186259.aspx
In the event of a failure, the log is replayed as you mention, but the situation regarding the data pages still being in memory and not written to disk does not matter, since the log of their modification is stored and can be retrieved.
It is not true that there is no real transaction, but if you are operating in simple logging mode then the ability to replay is not there.
For the integrity of the log file / same as the data file - a proper backup schedule and a proper restore testing schedule - do not just backup data / logs and assume they work.
What if the power failure happens after SQL Server issues a checkpoint and before the buffer cache is actuall written to disk? Isn't the content in buffer cache permanently missing?
The checkpoint start and end are different records on the transaction log.
The checkpoint is marked as succeeded only after the end of the checkpoint has been written into the log and the LSN of the oldest living transaction (including the checkpoint itself) is written into the database.
If the checkpoint fails to complete, the database is rolled back to the previous LSN, taking the data from the transaction log as necessary.
The transaction log is also stored as disk file, which is no different from the actual database file. So how could we guarantee the integrity of log file?
We couldn't. It's just the data are stored in two places rather than one.
If someone steals your server with both data and log files on it, your transactions are lost.