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Wednesday, December 30, 2009

ZFS logbias property

ZFS logbias property

ZFS Synchronous write bias in short logbias property is to provide a hint to ZFS about handling of synchronous requests in particular dataset. If logging is set to "latency" (the default) ZFS will use pool log devices i.e. ZIL (if configured) to handle the requests at low latency/delay. If logging is set to "throughput" then ZFS will not use configured pool log devices kind of a separated ZIL functionality of ZFS. ZFS will instead optimize synchronous operations for global pool throughput and efficient use of resources.

Basically ZFS logbias is proposed especially for managing performance of Oracle database.
Oracle manages two major types of files, the Data Files and the Redo Log files.
Writes to Redo Log files are in the course of all transactions and low latency/delay is a vital requirement. It is critical for good performance to give high priority to these write requests.
Data Files are also the subject of writes from DB writers as a form of scrubbing dirty blocks to insure DB buffer availability. Write latency/delay is much less an issue. Of more importance is achieving an acceptable level of throughput. These writes are less critical to delivered performance.

Both types of writes are synchronous (using O_DSYNC), and thus treated equally. They compete for the same resources: separate intent log, memory, and normal pool IO. The Data File writes slow down the overall potential performance of the critical Redo Log writers.

So in short - Redo Logs need short latencies/delays, so you would use zfs set logbias=latency oradbsrv1-redo/redologs. The data files of oracle need throughput, so you would configure zfs set logbias=throughput oradbsrv1-data/datafile.

If logbias is set to 'latency' (the default) then there is no change from the current implementation. If the logbias property is set to 'throughput' then intent log blocks will be allocated from the main pool instead of any separate intent log devices (if present). Also data will be written immediately to spread the write load thus making for quicker subsequent transaction group commits to the pool. This is the important change of setting logbias to throughput property.

For Oracle datafiles specifically, using the new setting of Synchronous write bias: Throughput has potential to deliver more stable performance in general and higher performance for redo log sensitive workloads.


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