VERITAS File System 4.1 Administrator's Guide

VxFS Performance: Creating, Mounting, and Tuning File Systems
I/O Tuning
Chapter 262
hsm_write_
prealloc
For a file managed by a hierarchical storage management
(HSM) application, hsm_write_prealloc preallocates disk
blocks before data is migrated back into the file system. An
HSM application usually migrates the data back through a
series of writes to the file, each of which allocates a few
blocks. By setting hsm_write_prealloc
(hsm_write_prealloc=1), a sufficient number of disk
blocks are allocated on the first write to the empty file so
that no disk block allocation is required for subsequent
writes. This improves the write performance during
migration.
The hsm_write_prealloc parameter is implemented
outside of the DMAPI specification, and its usage has
limitations depending on how the space within an
HSM-controlled file is managed. It is advisable to use
hsm_write_prealloc only when recommended by the HSM
application controlling the file system.
initial_extent_
size
Changes the default initial extent size. VxFS determines,
based on the first write to a new file, the size of the first
extent to be allocated to the file. Normally the first extent is
the smallest power of 2 that is larger than the size of the
first write. If that power of 2 is less than 8K, the first extent
allocated is 8K. After the initial extent, the file system
increases the size of subsequent extents (see
max_seqio_extent_size) with each allocation. Since 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 can change the default
initial extent size to be larger, so the doubling policy will
start from a much larger initial size and the file system will
not allocate a set of small extents at the start of file. Use
this parameter only on file systems that will have a very
large average file size. On these file systems it will result in
fewer extents per file and less fragmentation.
initial_extent_size is measured in file system blocks.
max_buf_data_size The maximum buffer size allocated for file data; either 8K
bytes or 64K bytes. Use the larger value for workloads
where large reads/writes are performed sequentially. Use
the smaller value on workloads where the I/O is random or
is done in small chunks. 8K bytes is the default value.