How do you set your Circular Logging?
Professionals: Disable Circular Logging 99% of the time
Amateurs: Think Circular Logging is the best thing since sliced bread
Litmus Test:
Why is Circular Logging a bad idea?
During disaster recovery, if you have over-written the transaction logs then
you can only restore as far as the last backup. When all the logs are
available, Exchange automatically replays the logs and recovers all the
transactions.
What is Circular Logging?
Exchange, SQL and Active directory databases all rely on transaction or
write-ahead logs. Events can be quickly written to the logs, then later
'committed' to the main database file. Circular logging over-writes these
transaction logs to save disk space.
Circular Logging
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No Circular Logging
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Log Numbers Disk Consumption
1 2 3 4 5
25 MB
2 3 4 5 6
25 MB
3 4 5 6 7
25 MB
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Log Numbers Disk Consumption
1 2 3 4 5
25 MB
1 2 3 4 5 6
30 MB
1 2 3 4 5 6 7
35 MB |
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How Transaction Logging Works
When a new piece of mail arrives, a transaction is written to the log.
If the server is busy there will be a delay before the information is committed
to the main EDB database file. A separate pointer, called a checkpoint
file,
maintains a record of how much of the log has been written into the EDB
database.
Disaster Recovery of Exchange Stores
Each Exchange Storage Group has one log for up to five individual stores.
One reason for having multiple stores is to reduce the time needed to restore in
disaster recovery conditions.
When disaster strikes and you have to restore an Exchange store you need a
full backup of the EDB database and all the logs since the the last backup.
So, if you allow circular logging to over-write the logs, then you lose
all data since the last backup. However, if you disable circular logging,
then you Exchange magically replays all the logs and brings back the Exchange
store to how it was before the disaster. This re-reading the logs is
called a soft recovery and happens automatically.
Where do you check the circular logging setting?
- Open the Exchange Administrator, double-click Servers.
- Select the server which has the storage group you want to enable circular
logging.
- Right-click the storage group, and then click Properties.
- On the General tab, click Enable circular logging, and then click Yes.
Why does such a potentially harmful setting exist?
The one time you may need circular logging is if your disk is full.
When you first install a database you always think 'no way will the disk ever
get full' - experience teaches it will!
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