Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

VxFS Performance: Creating, Mounting, and Tuning File Systems
I/O Tuning
Chapter 2 47
fcl_maxalloc Specifies the maximum amount of space that can be allocated to the
VxFS file change log (FCL). The FCL file is a sparse file that grows
as changes occur in the file system. When the space allocated to the
FCL file reaches the fcl_maxalloc value, the oldest FCL records
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 file
system runs out of space before fcl_maxalloc is reached, the FCL
is deactivated.
Either or both of the fcl_maxalloc or fcl_keeptime
parameters must be set before the file change log can be activated.
fcl_maxalloc does not apply to disk lay out Versions 1 through 5
fcl_winterval Specifies the time, in seconds, that must elapse before the VxFS file
change log (FCL) records a data overwrite, data extending write, or
data truncate for a file. The ability to limit the number of repetitive
FCL records for continuous writes to the same file is important for
file system performance and for applications processing the FCL.
fcl_winterval is best set to an interval less than the shortest
interval between reads of the FCL by any application. This way all
applications using the FCL can be assured of finding at least one
FCL record for any file experiencing continuous data changes.
fcl_winterval is enforced for all files in the file system.Each file
maintains its own time stamps, and the elapsed time between FCL
records is per file. This elapsed time can be overridden using the
VxFS FCL sync public API (see the vxfsu_fcl_sync(3) manual
page). fcl_winterval does not apply to disk layout Versions 1
through 5.