HP-UX HB v13.00 Ch-14 - JFS

HP-UX Handbook – Rev 13.00 Page 19 (of 47)
Chapter 14 Journaled File System (JFS)
October 29, 2013
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.
The 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 vxfs_fcl_sync(3) manual page.
fcl_winterval does not apply to disk layout Versions 1 through 5.
hsm_write_ prealloc
For a file managed by a hierarchical storage management (HSM) application, sm_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