HP-UX Reference (11i v2 07/12) - 1M System Administration Commands N-Z (vol 4)

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vxtunefs(1M) vxtunefs(1M)
block allocation is required for subsequent writes, which improves the write performance during
migration.
Because most applications write to files using a buffer size of 8K or less, the increasing extents start
doubling from a small initial extent. initial_extent_size
changes the default initial extent
size to a larger value, so the doubling policy starts from a much larger initial size, and the file system
won’t allocate a set of small extents at the start of file.
Use this parameter only on file systems that have a very large average file size. On such file systems,
there are fewer extents per file and less fragmentation.
initial_extent_size
is measured in file system blocks.
max_buf_data_size
Determines the maximum buffer size allocated for file data. The two accepted values are 8K bytes
and 64K bytes. The larger value can be beneficial for workloads where large reads/writes are per-
formed sequentially. The smaller value is preferable on workloads where the I/O is random or is done
in small chunks. The default value is 8K bytes.
max_direct_iosz
Maximum size of a direct I/O request issued by the file system. If there is a larger I/O request, it is
broken up into max_direct_iosz
chunks. This parameter defines how much memory an I/O
request can lock at once; do not set it to more than 20% of memory.
max_diskq
Limits the maximum disk queue generated by a single file. When the file system is flushing data for a
file and the number of pages being flushed exceeds
max_diskq, processes block until the amount of
data being flushed decreases. Although this does not limit the actual disk queue, it prevents syn-
chronizing processes from making the system unresponsive. The default value is 1 megabyte.
Although it does not limit the actual disk queue, max_diskq prevents processes that flush data to
disk, such as fsync, from making the system unresponsive.
See the write_throttle description for more information on pages and system memory.
max_seqio_extent_size
Increases or decreases the maximum size of an extent. When the file system is following its default
allocation policy for sequential writes to a file, it allocates an initial extent that is large enough for the
first write to the file. When additional extents are allocated, they are progressively larger (the algo-
rithm tries to double the size of the file with each new extent), so each extent can hold several writes
worth of data. This reduces the total number of extents in anticipation of continued sequential writes.
When there are no more writes to the file, unused space is freed for other files to use.
In general, this allocation stops increasing the size of extents at 2048 blocks, which prevents one file
from holding too much unused space.
max_seqio_extent_size
is measured in file system blocks.
read_ahead
In the absence of a specific caching advisory, the default for all VxFS read operations is to perform
sequential read ahead. The enhanced read ahead functionality implements an algorithm that allows
read aheads to detect more elaborate patterns (such as increasing or decreasing read offsets, or mul-
tithreaded file accesses) in addition to simple sequential reads. You can specify the following values
for read_ahead :
0 Disables read ahead functionality
1 Retains traditional sequential read ahead behavior
2 Enables enhanced read ahead for all reads
By default, read_ahead is set to 1, that is, VxFS detects only sequential patterns.
read_ahead detects patterns on a per-thread basis, up to a maximum of vx_era_nthreads.
The default number of threads is 5, however, you can change the default value by setting the
vx_era_nthreads parameter in the system configuration file, /etc/system .
read_nstream
The number of parallel read requests of size read_pref_io to have outstanding at one time. The
file system uses the product of read_nstream and read_pref_io to determine its read ahead
size. The default value for read_nstream is 1.
HP-UX 11i Version 2: December 2007 Update − 3 − Hewlett-Packard Company 673