JFS Tuning and Performance

cio

The cio option enables the file system for concurrent I/O.

Prior to VxFS 5.0.1, a separate license was needed to use the concurrent I/O feature. Beginning with

VxFS 5.0.1, concurrent I/O is available with the OnlineJFS license. Concurrent I/O is recommended

with applications which support its use.

remount

The remount option allows for a file system to be remounted online with different mount options. The

mount -o remount can be done online without taking down the application accessing the file system.

During the remount, all file I/O is flushed to disk and subsequent I/O operations are frozen until the

remount is complete. The remount option can result in a stall for some operations. Applications that

are sensitive to timeouts, such as Oracle Clusterware, should avoid having the file systems remounted

online.

Intent log options

There are 3 levels of transaction logging:

 tmplog – Most transaction flushes to the Intent Log are delayed, so some recent changes to the file

system will be lost in the event of a system crash.

 delaylog – Some transaction flushes are delayed.

 log – Most all structural changes are logged before the system call returns to the application.

Tmplog, delaylog, log options all guarantee the structural integrity of the file system in the event of a

system crash by replaying the Intent Log during fsck. However, depending on the level of logging,

some recent file system changes may be lost in the event of a system crash.

There are some common misunderstandings regarding the log levels. For read() and write() system

calls, the log levels have no effect. For asynchronous write() system calls, the log flush is always

delayed until after the system call is complete. The log flush will be performed when the actual user

data is written to disk. For synchronous write() systems calls, the log flush is always performed prior to

the completion of the write() system call. Outside of changing the file‟s access timestamp in the inode,

a read() system call makes no structural changes, thus does not log any information.

The following table identifies which operations cause the Intent Log to be written to disk synchronously

(flushed), or if the flushing of the Intent Log is delayed until sometime after the system call is complete

(delayed).