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

HP-UX Handbook – Rev 13.00 Page 18 (of 47)
Chapter 14 Journaled File System (JFS)
October 29, 2013
The number of parallel read requests of size read_pref_io to have outstanding at one time. The
file system uses the product of read_nstream multiplied by read_pref_io to determine its read
ahead size.
The default value for read_nstream is 1.
write_nstream
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.
discovered_direct_iosz
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.
fcl_keeptime
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.