GCSTART=n

If set to 1 then space recovery will become active on any emulated disks that have
free space. Normally space recovery will ignore emulated disks until they have
been updated. The default is 0.

Notes:

raq should be at least as large as ra. Read ahead threads are scheduled from entries in the read
ahead queue. Likewise rat should not exceed raq because only raq tracks or block groups can be
queued at any time. The number of writer threads wr should usually be 1 more than the number of host processors.
This is because one writer thread could be CPU-bound (compressing a track or block-group
image) and the other could be i/o-bound (writing the compressed image). The garbage collection interval governs the maximum time in seconds an updated track or block
group image will reside in storage before being written to the emulation file. A large value may
mean more data loss if a catastrophic error occurs. A small value may mean that more CPU time
is spent compressing images.

For example, suppose that a particular image is updated several times each second. If the inter-
val is changed from the default 5 seconds to 1 second, then that image will be compressed and
written 5 times more frequently. A large value may cause more cache flushes within a garbage
collection interval. These kinds of flushes mean that a read will wait because there are no avail-
able cache entries, slowing the emulated operating system. A large value will also cause more
pending free space to build up (since free space is flushed each interval). This may mean that the
garbage collector space recovery routine will perform more work and the resulting emulation file
may be larger. Specify fsync=1 and gcint=5 if you are seriously concerned about your data being lost due to a
failure. fsync will ensure your data on disk is coherent. However, fsync may cause a noticeable
performance degradation. Note that an fsync will not be performed more often than every 5
seconds.

8.24.4 Examples

Example 1:

Display the CCKD options panel.

HHC00013I Herc command: 'cckd opts'

HHC00346I cckd opts: comp=-1,compparm=-1,ra=2,raq=4,rat=2,wr=2,gcint=10

HHC00346I gcparm=0,nostress=0,freepend=-1,fsync=0,linuxnull=0,trace=0

Figure 47: CCKD OPTS command

Example 2:

Display the CCKD help panel.

HHC00013I Herc command: 'cckd help'

HHC00345I Command parameters for cckd:

HHC00345I help Display help message

HHC00345I stats Display cckd statistics

HHC00345I opts Display cckd options

HHC00345I comp=<n> Override compression (-1 ... 2)

HHC00345I compparm=<n> Override compression parm (-1 ... 9)

HHC00345I ra=<n> Set number readahead threads ( 1 ... 9)

HHC00345I raq=<n> Set readahead queue size ( 0 .. 16)

HHC00345I rat=<n> Set number tracks to read ahead ( 0 .. 16)

HHC00345I wr=<n> Set number writer threads ( 1 ... 9)

HHC00345I gcint=<n> Set garbage collector interval (sec) ( 1 .. 60)

HHC00345I gcparm=<n> Set garbage collector parameter (-8 ... 8)

HHC00345I gcstart Start garbage collector

HHC00345I nostress=<n> 1=Disable stress writes

HHC00345I freepend=<n> Set free pending cycles (-1 ... 4)

HHC00345I fsync=<n> 1=Enable fsync

HHC00345I linuxnull=<n> 1=Check for null linux tracks

HHC00345I trace=<n> Set trace table size ( 0 ... 200000)

Figure 48: CCKD HELP command

Example 3:

Display the CCKD statistics panel.

HHC00013I Herc command: 'cckd stats'

HHC00347I cckd stats:

HHC00347I reads.... 2726 Kbytes... 10997

HHC00347I writes... 141831 Kbytes... 179004

HHC00347I readaheads 751 misses... 65

HHC00347I syncios.. 1149 misses... 1352

HHC00347I switches. 3908 l2 reads. 322 strs wrt. 914

HHC00347I cachehits 1929 misses... 1979

HHC00347I l2 hits.. 15250 misses... 322

HHC00347I waits............ i/o...... 158 cache.... 0

HHC00347I garbage collector moves.... 984 Kbytes... 173612

Figure 49: CCKD STATS command

Example 4:

Set a CCKD parameter.

HHC00013I Herc command: 'cckd gcint=30'

HHC00346I cckd opts: comp=-1,compparm=-1,ra=2,raq=4,rat=2,wr=2,gcint=30

HHC00346I gcparm=0,nostress=0,freepend=-1,fsync=0,linuxnull=0,trace=0

Figure 50: CCKD command (set options)

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