Veritas File System 5.0 AdministratorÆs Guide, HP-UX 11i v3, First Edition, May 2008

Table 2-1
Tunable VxFS I/O parameters (continued)
DescriptionParameter
The number of parallel write requests of size
write_pref_io to have outstanding at one time. The
file system uses the product of write_nstream
multiplied by write_pref_io to determine when to do
flush behind on writes. The default value for
write_nstream is 1.
write_nstream
Any file I/O requests larger than
discovered_direct_iosz are handled as discovered
direct I/O. A discovered direct I/O is unbuffered similar
to direct I/O, but it does not require a synchronous commit
of the inode when the file is extended or blocks are
allocated. For larger I/O requests, the CPU time for
copying the data into the page cache and the cost of using
memory to buffer the I/O data becomes more expensive
than the cost of doing the disk I/O. For these I/O requests,
using discovered direct I/O is more efficient than regular
I/O. The default value of this parameter is 256K.
discovered_direct_iosz
Specifies the minimum amount of time, in seconds, that
the VxFS File Change Log (FCL) keeps records in the log.
When the oldest 8K block of FCL records have been kept
longer than the value of fcl_keeptime, they are purged
from the FCL and the extents nearest to the beginning of
the FCL file are freed. This process is referred to as
“punching a hole.” Holes are punched in the FCL file in
8K chunks.
If the fcl_maxalloc parameter is set, records are purged
from the FCL if the amount of space allocated to the FCL
exceeds fcl_maxalloc, even if the elapsed time the
records have been in the log is less than the value of
fcl_keeptime. If the file system runs out of space before
fcl_keeptime is reached, the FCL is deactivated.
Either or both of the fcl_keeptime or fcl_maxalloc
parameters must be set before the File Change Log can be
activated. fcl_keeptime does not apply to disk layout
Versions 1 through 5.
fcl_keeptime
VxFS performance: creating, mounting, and tuning file systems
Tuning I/O
46