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"Fast CCW translation" is related to a CP feature where CP will use a more efficient path in the translation of virtual to real addresses associated with CCWs and untranslation (real to virtual)., It is only available for a subset of DASD I/O. It should not be confused with bypassing of CCW translation.

"CCW translation bypass" can be done for V=R guests for dedicated devices and, in limited cases, full-pack minidisks. It can be controlled by the SET CCWTRAN command. For a V=R machine the default for CCWTRAN is set OFF. CCWTRAN can not be set OFF for a V=F machine. If you SET CCWTRAN ON for a V=R machine, then the benefit of I/O Assist is lost. In the past, CCW translation bypass was often referred to as I/O fast path.

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 Virtual machine characteristics          |   Performance Benefit
------------------------------------------|------------------------------
  Mach  Dedicated    SET       SET        |   I/O      CCW    Fast CCW
  Type   DASD (1)  IOASSIST  CCWTRAN      |   Assist   Bypass Translation
------------------------------------------|------------------------------
  V=R      Yes       ON        OFF        |    Yes     Yes      n/a (2)
  V=R      Yes       ON        ON         |    No      No       Yes (3)
  V=R      Yes       OFF       OFF        |    No      Yes      n/a (2)
  V=R      Yes       OFF       ON         |    No      No       Yes (3)
  V=R      No        ON        OFF        |    No      Yes (4)  No  (4)
  V=R      No        OFF       OFF        |    No      Yes (4)  No  (4)
  V=R      No        ON        ON         |    No      No       Yes
  V=R      No        OFF       ON         |    No      No       Yes
 
  V=F      Yes       ON        n/a (5)    |    Yes     No       Yes (6)
  V=F      Yes       OFF       n/a (5)    |    No      No       Yes
  V=F      No        ON        n/a (5)    |    No      No       Yes
  V=F      No        OFF       n/a (5)    |    No      No       Yes

1. Dedicated DASD is required for I/O Assist.
2. There's no need for any CCW translation in this scenario, and thus fast-translation is not applicable.
3. This setting of controls can be used when CCW translation is explicitly required, but should not be otherwise used. I/O Assist is lost when CCWTRAN is set ON for a V=R guest.
4. Minidisks for a V=R guest fall into two categories:
   1. Non-fullpack. This group will always go through CCW translation and therefore will receive a benefit from CCW Fast Path.
   2. Fullpack minidisks for a V=R guest with CCWTRANS=OFF can bypass CCW translation when the channel program starts with SET FILEMASK and FULL SEEK (or DEFINE EXTENT and LOCATE or LOCATE RECORD). Note also that enhanced minidisk cache will not be in effect. In fact bypass CCW translation may cause a purge of associated data in minidisk cache for the given minidisk.
5. For V=F machines CCWTRANS can not be set off, so that switch doesn't apply.
6. CCW Fast Path translation may provide a benefit if the device happen s to be out of I/O ASSIST at the time of the SSCH/SIOF.
7. CCW fast path translation is slightly slower for V=R due to requirements for V=R Recoverability.
8. In an LPAR environment, I/O Assist is not available. CCW translation bypass requires the address-checking-limit facility which is not available in LPAR configurations. V=F guests can not be used with LPAR.

-------------------------------------------------------------------------
CCW Fastpath Translation Matrix - Time frame of enhancements
-------------------------------------------------------------------------
                   VM/ESA 1.1.1
                  w/ APAR 51012   VM/ESA 1.2.0  VM/ESA 1.2.1
V=V Full Pack           Yes          Yes           Yes
V=V Partial Pack        Yes          Yes           Yes
V=F Full Pack           Yes          Yes           Yes
V=F Partial Pack        Yes          Yes           Yes
V=R Full Pack           No (a)       No (a)        Yes
V=R Partial Pack        Yes          Yes           Yes
 
Notes:
   a - there are a few rare cases where fast path may occur
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For additional information on I/O Assist see the following:

• VM/ESA Performance SC24-5642-01 under "I/O Interpretation Facilities" in Chapter 3. (Manual must be suffix 01 or higher). Contains requirements for I/O Assist, reasons for dropping out of I/O Assist, and information on available data.
• VM/ESA Release 2 Performance Report GG66-3245-00 - superset of above information, also includes example performance reports for different IBM products.

For additional information on Fast CCW Translation see the following:

• Washington System Center Flash 9220 "VM/ESA 1.1 CCW Translation Performance Improvements".
• VM/ESA Release 1.1 Performance Report, GG66-3236.
• VM/ESA Release 2 Performance Report, GG66-3245.
• VM/ESA Release 2.1 Performance Report, GC24-5673-00.
• Counters on Fast CCW translation are found in RTM/ESA on DISPLAY SYSTEM screen. FTR_ABOR, FTR_DONE, FTR_NE, and FTR_TOTL.
• Counters can also be found by CP LOCATE HCPFTRRW which points to 3 fullword running counters:
• Number of fast CCW translations done
• Number of fast CCW translations started and then aborted
• Number of CCW translations, not eligible for fast trans

Update, January 7, 2005

z/VM 5.1.0 drops support for preferred guests and for I/O Assist. This has implications for CCW translation, especially in processor-constrained environments. For more information, visit this tip.