How can the LockList configuration parameter be adjusted to minimize the potential for DB2 deadlocks in a Tivoli Storage Manager environment?
If the DB2 LOCKLIST parameter is managed incorrectly, it may cause deadlocks to be returned to the Tivoli Storage Manager application for insert, delete and update requests.
DB2 LOCKLIST PARAMETER
The DB2 LOCKLIST parameter indicates the amount of storage that is allocated within the Database Heap to store lock information. There is one LockList per database and it contains the locks held by all applications, or in this case Tivoli Storage Manager Server transactions, concurrently connected to the database. Locks are stored and manipulated by DB2 in the form of a request block which consumes 40 bytes of memory on 32-bit platforms or 64 bytes on 64-bit systems. By default, DB2 manages the LockList size automatically and increases or decreases the total LockList allocation based on current activity. To determine the current value, issue the following command:
db2 "get db cfg for <db name>"
The LockList is comprised of memory allocated as 4K pages. A LOCKLIST parameter value of 10000 (4KB pages) is equivalent to 40,960,000bytes (about 39MB) of memory allotted to the LockList from the Database Heap. A single lock request may require up to 128 bytes out of the LockList heap as one request block is used for the lock descriptor and the second block is used to associate the Tivoli Storage Manager server transaction with the resource being locked. If the lock resource has already been requested then only one request block is required for future transactions requesting that lock resource.
IMPACT TO THE TIVOLI STORAGE MANAGER SERVER
The proper management of the DB2 LockList is crucial to prevent deadlocks within the Tivoli Storage Manager server. When the server is under average workload and the amount of concurrent activity is not driving large spikes of lock requests, the DB2 automatic handling of the LockList works well for the Tivoli Storage Manager server. However, when the amount of concurrent data movement activity is high, the inherent spikes to the DB2 LockList can cause lock escalations which will ultimately result in deadlocks within the server.
The best case scenario is that the Tivoli Storage Manager server will re-drive the request and the subsequent operation will not fail due to the deadlock. Most Tivoli Storage Manager server operations effectively recover from this but in the case where a lot of data had been moved, deleted, or updated, the time required to process the data a second, or even third time can result in severe performance degradation on the Tivoli Storage Manager server. The worst case scenario is that the operation cannot be retried and the entire operation fails.
If the amount of concurrent data being moved exceeds 500GB at any given time, it is recommended that the DB2 LOCKLIST parameter be adjusted as explained below. If the Tivoli Storage Manager server is involved with deduplicating large files either from the client, or server, properly tuning the LockList manually is extremely critical as large file deduplication inherently drives the number of rows needing to be managed by DB2 very high. This is because a large file is managed in many small pieces which result in many row locks within the DB2 LockList.
The following Tivoli Storage Manager server messages are issued when a transaction is being rolled back due to a deadlock within DB2.
ANR0159E dbieval.c(863): Database deadlock detected on XX:X.
ANR0162W Supplemental database diagnostic information:
-X:XXXXX:-XXX ([IBM][CLI Driver][DB2/XXXXX] SQL0911N
The current transaction has been rolled back because of a
deadlock or timeout. Reason code "2". SQLSTATE=40001).
TUNING THE DB2 LOCKLIST
To choose a LOCKLIST parameter value that will minimize the amount of deadlocks that may occur within a Tivoli Storage Manager server environment, the maximum amount of data that can be moved, or managed, within the Tivoli Storage Manager server at any given time must be known or at least approximated with a fair amount of precision. .
Tuning Best Practice: Increase the DB2 LOCKLIST value by twice the amount of anticipated workload so that there are no unexpected failures during daily and nightly operations.
It is important to note that increasing the DB2 LOCKLIST parameter value has a direct effect on the amount of memory that is required within the instance memory region. Increasing the parameter should be done with the understanding of what the potential memory implications will be on the system. The following information and examples can be used to tune the DB2 LOCKLIST parameter to an appropriate value based on the Tivoli Storage Manager server environment and workload:
KEY FORMULA FOR DB2 LOCKLIST ADJUSTMENTS
Concurrent data movement
500GB = 2,500,000 locks = 122000 LOCKLIST
1TB = 5,000,000 locks = 244000 LOCKLIST
5TB = 25,000,000 locks = 1220000 LOCKLIST
MEMORY REQUIREMENTS FOR LOCKLIST
122000 LOCKLIST = 499MB (122000 X 4096)
244000 LOCKLIST = 1GB (244000 X 4096)
1220000 LOCKLIST = 4.9 GB (1220000 X 4096)
100 Tivoli Storage Manager clients store 1TB of data in a 4 hour nightly window
Migration has the ability to move 1TB of data in the same 4 hour window
Total concurrent data movement - 2TB
DB2 LOCKLIST recommendation - 588000
Additional impact to system memory (potential) - 2.4 GB
Migration has the ability to move up to 1TB of data in an 8 hour window
Reclamation can run in the same window and process up to 2TB of data
Expiration runs in the same window and can process up to 10 Million files which represent 500GB of data
Total concurrent data movement/management - 3.5 TB
DB2 LOCKLIST recommendation - 954000
Additional impact to system memory (potential) - 3.9GB
UPDATING THE DB2 LOCKLIST PARAMETER
The following command can be used to update the LOCKLIST parameter:
db2 "connect to <db name>"
db2 "update db cfg for <db name> using LOCKLIST XXXXXXXX immediate"
where XXXXXXXXX = LOCKLIST value
The new value can be verified by issuing the following command:
db2 "get db cfg for <db name> show detail"
NOTE: As a result of updating the DB2 LOCKLIST parameter the MAXLOCKS parameter will be set to manual as well. The MAXLOCKS parameter controls what percentage of the LockList that a single transaction can own at any given time. Please ensure this value is set from 95-100. If it is not in this range, the parameter can be updated with the following command:
db2 "connect to <db name>"
db2 "update db cfg for <db name> using maxlocks 97 immediate"