Veritas File System 5.0 Administrator's Guide (September 2006)
Table 2-1
Tunable VxFS I/O parameters (continued)
DescriptionParameter
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.
hsm_write_ prealloc
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 with each
allocation.
See max_seqio_extent_size.
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 starts from a much larger initial size and the file
system does not 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 these file systems it
results in fewer extents per file and less fragmentation.
initial_extent_size is measured in file system
blocks.
initial_extent_size
VxFS performance: creating, mounting, and tuning file systems
Tuning I/O
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