Fsync-ing the write ahead log in sync threading

Monday, September 27, Database speed tests mysql and postgresql - part 1 There has been major changes in mysql and postgres over a couple of years. Mysql has been focusing on improving and optimizing innodb. Postgres on the other hand has been focusing on database replication and hot standby. Recently postgres came out with version 9.

Fsync-ing the write ahead log in sync threading

Overview The default method by which SQLite implements atomic commit and rollback is a rollback journal. Beginning with version 3. There are advantages and disadvantages to using WAL instead of a rollback journal. WAL is significantly faster in most scenarios.

WAL provides more concurrency as readers do not block writers and a writer does not block readers. Reading and writing can proceed concurrently.

WAL uses many fewer fsync operations and is thus less vulnerable to problems on systems where the fsync system call is broken. But there are also disadvantages: All processes using a database must be on the same host computer; WAL does not work over a network filesystem.

Transactions that involve changes against multiple ATTACHed databases are atomic for each individual database, but are not atomic across all databases as a set. You must be in a rollback journal mode to change the page size. It is not possible to open read-only WAL databases.

fsync-ing the write ahead log in sync threading

The opening process must have write privileges for "-shm" wal-index shared memory file associated with the database, if that file exists, or else write access on the directory containing the database file if the "-shm" file does not exist. There is an additional quasi-persistent "-wal" file and "-shm" shared memory file associated with each database, which can make SQLite less appealing for use as an application file-format.

There is the extra operation of checkpointing which, though automatic by default, is still something that application developers need to be mindful of. WAL works best with smaller transactions. WAL does not work well for very large transactions.

For transactions larger than about megabytes, traditional rollback journal modes will likely be faster. It is recommended that one of the rollback journal modes be used for transactions larger than a few dozen megabytes.

How WAL Works The traditional rollback journal works by writing a copy of the original unchanged database content into a separate rollback journal file and then writing changes directly into the database file. In the event of a crash or ROLLBACKthe original content contained in the rollback journal is played back into the database file to revert the database file to its original state.

The WAL approach inverts this. The original content is preserved in the database file and the changes are appended into a separate WAL file. Thus a COMMIT can happen without ever writing to the original database, which allows readers to continue operating from the original unaltered database while changes are simultaneously being committed into the WAL.

Multiple transactions can be appended to the end of a single WAL file. Checkpointing Of course, one wants to eventually transfer all the transactions that are appended in the WAL file back into the original database.ZK fsync warning.

Have you seen this before? , [myid:1] - WARN [[email protected]] - fsync-ing the write ahead log in SyncThread:1 took .

log says , [myid:1] - WARN [[email protected]] - fsync-ing the write ahead log in SyncThread:1 took ms which will adversely effect operation latency. See the ZooKeeper troubleshooting guide. In a traditional "write-ahead log + main storage area" setting, I would expect that – for each "operation", e.g., an insertion into a table plus the corresponding changes to indexes etc – first the log is written, before any of the corresponding changes to the main storage area are written.

The symptoms are: (1) All nodes show messages like "fsync-ing the write ahead log in SyncThread:0 took ms which will adversely effect operation latency. See the ZooKeeper troubleshooting guide" with times typically around 5 seconds.

ZK fsync warning

[ ,] INFO Processed session termination for sessionid: 0x15aa58d (metin2sell.comquestProcessor) [ ,] WARN fsync-ing the write ahead log in SyncThread:0 took ms which will adversely effect operation latency. Beginning with version (), a new "Write-Ahead Log" option (hereafter referred to as "WAL") is available.

There are advantages and disadvantages to using WAL instead of a rollback journal.

Zookeeper f-sync issue resulting in Kafka timeout - Stack Overflow