VERITAS File System 4.1 Administrator's Guide

VxFS Performance: Creating, Mounting, and Tuning File Systems
I/O Tuning
Chapter 2 61
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.