JFS Tuning and Performance

Data access methods

Buffered/cached I/O

By default, most access to files in a VxFS file system is through the cache. In HP-UX 11i v2 and

earlier, the HP-UX Buffer Cache provided the cache resources for file access. In HP-UX 11i v3 and

later, the Unified File Cache is used. Cached I/O allows for many features, including asynchronous

prefetching known as

read ahead

, and asynchronous or delayed writes known as

flush behind

Read ahead

When reads are performed sequentially, VxFS detects the pattern and reads ahead or prefetches data

into the buffer/file cache. VxFS attempts to maintain 4 read ahead “segments” of data in the

buffer/file cache, using the read ahead size as the size of each segment.

Figure 1. VxFS read ahead

The segments act as a “pipeline”. When the data in the first of the 4 segments is read, asynchronous

I/O for a new segment is initiated, so that 4 segments are maintained in the read ahead pipeline.

The initial size of a read ahead segment is specified by VxFS tunable read_pref_io. As the process

continues to read a file sequentially, the read ahead size of the segment approximately doubles, to a

maximum of read_pref_io * read_nstream.

For example, consider a file system with read_pref_io set to 64 KB and read_nstream set to 4. When

sequential reads are detected, VxFS will initially read ahead 256 KB (4 * read_pref_io). As the

process continues to read ahead, the read ahead amount will grow to 1 MB (4 segments *

read_pref_io * read_nstream). As the process consumes a 256 KB read ahead segment from the front

of the pipeline, asynchronous I/O is started for the 256 KB read ahead segment at the end of the

pipeline.

By reading the data ahead, the disk latency can be reduced. The ideal configuration is the minimum

amount of read ahead that will reduce the disk latency. If the read ahead size is configured too large,

the disk I/O queue may spike when the reads are initiated, causing delays for other potentially more

critical I/Os. Also, the data read into the cache may no longer be in the cache when the data is

needed, causing the data to be re-read into the cache. These cache misses will cause the read ahead

size to be reduced automatically.

Note that if another thread is reading the file randomly or sequentially, VxFS has trouble with the

sequential pattern detection. The sequential reads may be treated as random reads and no read

ahead is performed. This problem can be resolved with enhanced read ahead discussed later. The

read ahead policy can be set on a per file system basis by setting the read_ahead tunable using

vxtunefs(1M). The default read_ahead value is 1 (normal read ahead).

256K

Sequential read