HP-UX VxFS tuning and performance - filibeto.org
HP-UX VxFS tuning and performance - filibeto.org HP-UX VxFS tuning and performance - filibeto.org
6 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 256K 256K 256K 256K 256K Sequential read 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).
Read ahead with VxVM stripes The default value for read_pref_io is 64 KB, and the default value for read_nstream is 1, except when the file system is mounted on a VxVM striped volume where the tunables are defaulted to match the striping attributes. For most applications, the 64 KB read ahead size is good (remember, VxFS attempts to maintain 4 segments of read ahead size). Depending on the stripe size (column width) and number of stripes (columns), the default tunables on a VxVM volume may result in excessive read ahead and lead to performance problems. Consider a VxVM volume with a stripe size of 1 MB and 20 stripes. By default, read_pref_io is set to 1 MB and read_nstream is set to 20. When VxFS initiates the read ahead, it tries to prefetch 4 segments of the initial read ahead size (4 segments * read_pref_io). But as the sequential reads continue, the read ahead size can grow to 20 MB (read_pref_io * read_nstream), and VxFS attempts to maintain 4 segments of read ahead data, or 80 MB. This large amount of read ahead can flood the SCSI I/O queues or internal disk array queues, causing severe performance degradation. Note If you are using VxVM striped volumes, be sure to review the read_pref_io and read_nstream values to be sure excessive read ahead is not being done. False sequential I/O patterns and read ahead Some applications trigger read ahead when it is not needed. This behavior occurs when the application is doing random I/O by positioning the file pointer using the lseek() system call, then performing the read() system call, or by simply using the pread() system call. Figure 2. False sequential I/O patterns and read ahead If the application reads two adjacent blocks of data, the read ahead algorithm is triggered. Although only 2 reads are needed, VxFS begins prefetching data from the file, up to 4 segments of the read ahead size. The larger the read ahead size, the more data read into the buffer/file cache. Not only is excessive I/O performed, but useful data that may have been in the buffer/file cache is invalidated so the new unwanted data can be stored. Note If the false sequential I/O patterns cause performance problems, read ahead can be disabled by setting read_ahead to 0 using vxtunefs(1M), or direct I/O could be used. Enhanced read ahead 8K 8K 8K lseek()/read()/read() lseek()/read() Enhanced read ahead can detect non-sequential patterns, such as reading every third record or reading a file backwards. Enhanced read ahead can also handle patterns from multiple threads. For 7
- Page 1 and 2: HP-UX VxFS tuning and performance T
- Page 3 and 4: Understanding VxFS In order to full
- Page 5: File system reorganization attempts
- Page 9 and 10: Figure 4. Flush behind As data is w
- Page 11 and 12: The read flush behind feature has t
- Page 13 and 14: Note The OnlineJFS license is requi
- Page 15 and 16: Concurrent I/O The main problem add
- Page 17 and 18: Zero Filled On Demand (ZFOD) extent
- Page 19 and 20: Table 3. Default intent log size FS
- Page 21 and 22: Using datainlog has no affect on no
- Page 23 and 24: Table 4. Intent log flush behavior
- Page 25 and 26: ead_ahead The default read_ahead va
- Page 27 and 28: Unified File Cache on HP-UX 11i v3
- Page 29 and 30: Note When setting vx_ninode to redu
- Page 31 and 32: Patches Performance problems are of
Read ahead with VxVM stripes<br />
The default value for read_pref_io is 64 KB, <strong>and</strong> the default value for read_nstream is 1, except when<br />
the file system is mounted on a VxVM striped volume where the tunables are defaulted to match the<br />
striping attributes. For most applications, the 64 KB read ahead size is good (remember, <strong>VxFS</strong><br />
attempts to maintain 4 segments of read ahead size). Depending on the stripe size (column width)<br />
<strong>and</strong> number of stripes (columns), the default tunables on a VxVM volume may result in excessive read<br />
ahead <strong>and</strong> lead to <strong>performance</strong> problems.<br />
Consider a VxVM volume with a stripe size of 1 MB <strong>and</strong> 20 stripes. By default, read_pref_io is set to<br />
1 MB <strong>and</strong> read_nstream is set to 20. When <strong>VxFS</strong> initiates the read ahead, it tries to prefetch 4<br />
segments of the initial read ahead size (4 segments * read_pref_io). But as the sequential reads<br />
continue, the read ahead size can grow to 20 MB (read_pref_io * read_nstream), <strong>and</strong> <strong>VxFS</strong> attempts<br />
to maintain 4 segments of read ahead data, or 80 MB. This large amount of read ahead can flood<br />
the SCSI I/O queues or internal disk array queues, causing severe <strong>performance</strong> degradation.<br />
Note<br />
If you are using VxVM striped volumes, be sure to review the<br />
read_pref_io <strong>and</strong> read_nstream values to be sure excessive read<br />
ahead is not being done.<br />
False sequential I/O patterns <strong>and</strong> read ahead<br />
Some applications trigger read ahead when it is not needed. This behavior occurs when the<br />
application is doing r<strong>and</strong>om I/O by positioning the file pointer using the lseek() system call, then<br />
performing the read() system call, or by simply using the pread() system call.<br />
Figure 2. False sequential I/O patterns <strong>and</strong> read ahead<br />
If the application reads two adjacent blocks of data, the read ahead algorithm is triggered. Although<br />
only 2 reads are needed, <strong>VxFS</strong> begins prefetching data from the file, up to 4 segments of the read<br />
ahead size. The larger the read ahead size, the more data read into the buffer/file cache. Not only is<br />
excessive I/O performed, but useful data that may have been in the buffer/file cache is invalidated so<br />
the new unwanted data can be stored.<br />
Note<br />
If the false sequential I/O patterns cause <strong>performance</strong> problems, read<br />
ahead can be disabled by setting read_ahead to 0 using vxtunefs(1M), or<br />
direct I/O could be used.<br />
Enhanced read ahead<br />
8K 8K 8K<br />
lseek()/read()/read() lseek()/read()<br />
Enhanced read ahead can detect non-sequential patterns, such as reading every third record or<br />
reading a file backwards. Enhanced read ahead can also h<strong>and</strong>le patterns from multiple threads. For<br />
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