JFS Tuning and Performance

example, two threads can be reading from the same file sequentially and both threads can benefit

from the configured read ahead size.

Figure 3. Enhanced read ahead

Enhanced read ahead can be set on a per file system basis by setting the read_ahead tunable to 2

with vxtunefs(1M).

Read ahead on 11i v3

With the introduction of the Unified File Cache (UFC) on HP-UX 11i v3, significant changes were

needed for VxFS. As a result, a defect was introduced which caused poor VxFS read ahead

performance. The problems are fixed in the following patches according to the VxFS version:

 PHKL_40018 - 11.31 vxfs4.1 cumulative patch

 PHKL_41071 - 11.31 VRTS 5.0 MP1 VRTSvxfs Kernel Patch

 PHKL_41074 - 11.31 VRTS 5.0.1 GARP1 VRTSvxfs Kernel Patch

These patches are critical to VxFS read ahead performance on 11i v3.

Another caveat with VxFS 4.1 and later occurs when the size of the read() is larger than the

read_pref_io value. A large cached read can result in a read ahead size that is insufficient for the

read. To avoid this situation, the read_pref_io value should be configured to be greater than or equal

to the application read size.

For example, if the application is doing 128 KB read() system calls, then the application can

experience performance issues with the default read_pref_io value of 64 KB, as the VxFS would only

read ahead 64 KB, leaving the remaining 64 KB that would have to be read in synchronously.

Note

On HP-UX 11i v3, the read_pref_io value should be set to the size of the

largest logical read size for a process doing sequential I/O through cache.

The write_pref_io value should be configured to be greater than or equal to the read_pref_io value

due to a new feature on 11i v3 called

read flush behind

, which will be discussed later.

Flush behind

During normal asynchronous write operations, the data is written to buffers in the buffer cache, or

memory pages in the file cache. The buffers/pages are marked “dirty” and control returns to the

application. Later, the dirty buffers/pages must be flushed from the cache to disk.

There are 2 ways for dirty buffers/pages to be flushed to disk. One way is through a “sync”, either

by a sync() or fsync() system call, or by the syncer daemon, or by one of the vxfsd daemon threads.

The second method is known as “flush behind” and is initiated by the application performing the

writes.

64K

128K

Patterned Read