Document revision date: 30 March 2001

XFC uses a technique called read-ahead caching to improve the performance of applications that read data sequentially. It detects when a file is being read sequentially in equal-sized I/Os, and fetches data ahead of the current read, so that the next read instruction can be satisfied from cache.

To disable read-ahead caching on the local node, set the dynamic system parameter VCC_READAHEAD to 0. By default, this parameter is 1, which allows the local node to use read-ahead caching.

Example

This example disables read-ahead caching on the local node:

$ RUN SYS$SYSTEM:SYSGEN SYSGEN> USE CURRENT SYSGEN> SET VCC_READAHEAD 0 SYSGEN> WRITE ACTIVE

This series of commands affects volumes currently mounted on the local node, as well as volumes mounted in the future. Once you enter these commands, read-ahead caching is not used on the local node.

18.5.5 Monitoring Performance

XFC provides more information than VIOC. For example, you can obtain information on system-wide, volume-wide, or even a per-file basis. Disk I/O response times are also available. See the OpenVMS DCL Dictionary: N--Z for a description of the SHOW MEMORY command.

18.5.5.1 System-Wide Statistics

Use SHOW MEMORY /CACHE to monitor the overall system performance of XFC. For example:

$ SHOW MEMORY /CACHE System Memory Resources on 26-JAN-2001 15:58:18.71 Extended File Cache (Time of last reset: 24-JAN-2001 15:03:39.05) Allocated (Mbytes) (1) 3000.00 Maximum size (Mbytes) (11) 5120.00 Free (Mbytes) (2) 2912.30 Minimum size (Mbytes) (12) 3000.00 In use (Mbytes) (3) 87.69 Percentage Read I/Os (13) 98% Read hit rate (4) 92% Write hit rate (14) 0% Read I/O count (5) 178136 Write I/O count (15) 1867 Read hit count (6) 165470 Write hit count (16) 0 Reads bypassing cache (7) 2802 Writes bypassing cache (17) 39 Files cached open (8) 392 Files cached closed (18) 384 Vols in Full XFC mode (9) 0 Vols in VIOC Compatible mode (19) 4 Vols in No Caching mode (10) 1 Vols in Perm. No Caching mode (20) 0

(1) Allocated	The amount of memory currently allocated to the cache.
(2) Free	The amount of memory currently allocated to the cache that is not being used.
(3) In Use	The amount of memory currently allocated to the cache that is being used. This is the difference between the Allocated value and the Free value.
(4) Read hit rate	The ratio of the Read hit count field divided by the Read I/O count field.
(5) Read I/O count	The total number of read I/Os seen by the cache since system startup.
(6) Read hit count	The total number of read hits since system startup. A read hit is a read I/O that did not require a physical I/O to disk because the data was found in the cache.
(7) Reads bypassing cache	The total number of read I/Os since system startup that were seen by the cache but were not cached, for example, because they were too big, or they were for volumes mounted /NOCACHE, or they specified one of the following QIO modifiers: IO$M_DATACHECK, IO$M_INHRETRY, or IO$M_NOVCACHE.
(8) Files cached open	The number of open files currently being cached.
(9) Volumes in Full XFC mode	This is 0 if caching is disabled either on the local node or on another node in the OpenVMS Cluster. If caching is not disabled on any node, this is the number of volumes that are mounted on the local node and that satisfy both of these criteria: The volume is not mounted /NOCACHE on the local node or any other node in the OpenVMS Cluster. The volume is mounted only on nodes that are using XFC.
(10) Volumes in No Caching mode	If caching is disabled on the local node or on another node in the OpenVMS Cluster, this is the number of volumes that are currently mounted on the local node. Otherwise it is zero.
(11) Maximum size	The maximum size that the cache could ever grow to.
(12) Minimum size	The minimum size that the cache could ever shrink to. This is controlled by the value of the VCC$MIN_CACHE_SIZE entry in the reserved memory registry.
(13) Percentage Read I/Os	Percentage of I/Os that are reads.
(14) Write hit rate	This field is reserved for future use.
(15) Write I/O count	The total number of write I/Os seen by the cache since system startup.
(16) Write hit count	This field is reserved for future use.
(17) Writes bypassing cache	The total number of write I/Os since system startup that were seen by the cache but were not cached, for example, because they were too big, or they were for volumes mounted /NOCACHE, or they specified one of the following QIO modifiers: IO$M_DATACHECK, IO$M_ERASE, IO$M_INHRETRY, or IO$M_NOVCACHE.
(18) Files cached closed	The number of closed files that still have valid data in the cache.
(19) Volumes in VIOC compatible mode	This is 0 if caching is disabled either on the local node or on another node in the OpenVMS Cluster. If caching is not disabled on any node, this is the number of volumes that are mounted on the local node and that satisfy either of these criteria: The volume is mounted /NOCACHE either on the local node or on another node in the OpenVMS Cluster. The volume is mounted on a node in the OpenVMS Cluster that is using VIOC. The files in these volumes can't be cached when they are being shared for writing in an OpenVMS Cluster.
(20) Vols in Perm. No Caching mode	This field should be zero. If nonzero, XFC has detected an illegal write operation to this device and has disabled caching to this device.

See the OpenVMS DCL Dictionary: N--Z for a description of the SHOW MEMORY command.

18.5.6 Using XFC in a Mixed Architecture OpenVMS Cluster

In an OpenVMS Cluster, some nodes can use XFC and other nodes can use VIOC. This allows mixed architecture clusters to benefit from XFC.

When a volume is mounted on a node that is using VIOC, the nodes using XFC cannot cache any files in the volume that are shared for writing. A file that is shared for writing is one that is being accessed by more than one node in an OpenVMS Cluster, and at least one of those nodes opened it for write access.

18.6 Managing the Virtual I/O Cache

This section describes how to manage VIOC. It describes the following tasks:

Task	Section
Selecting VIOC on an Alpha system	Section 18.6.2
Controlling the size of the cache	Section 18.6.3
Monitoring performance	Section 18.6.4

The virtual I/O cache is a clusterwide, write-through, file-oriented, disk cache that can reduce the number of disk I/O operations and increase performance. The purpose of the virtual I/O cache is to increase system throughput by reducing file I/O response times with minimum overhead. The virtual I/O cache operates transparently of system management and application software, and maintains system reliability while it significantly improves virtual disk I/O read performance.

18.6.1 Understanding How the Cache Works

The virtual I/O cache can store data files and image files. For example, ODS-2 disk file data blocks are copied to the virtual I/O cache the first time they are accessed. Any subsequent read requests of the same data blocks are satisfied from the virtual I/O cache (hits) eliminating any physical disk I/O operations (misses) that would have occurred.

Depending on your system work load, you should see increased application throughput, increased interactive responsiveness, and reduced I/O load.

Several policies govern how the cache manipulates data, as follows:

• Write-through---All write I/O requests are written to the cache as well as to the disk.
• Least Recently Used (LRU)---If the cache is full, the least recently used data in the cache is replaced.
• Cached data maintained across file close---Data remains in the cache after a file is closed.
• Allocate on read and write requests---Cache blocks are allocated for read and write requests.

If for some reason, you want an Alpha system to use VIOC instead of XFC, follow these steps:

1. Remove the entry for VCC$MIN_CACHE_SIZE from the reserved memory registry, using the Sysman utility's RESERVED_MEMORY REMOVE command:

   $ RUN SYS$SYSTEM:SYSMAN SYSMAN> RESERVED_MEMORY REMOVE VCC$MIN_CACHE_SIZE /NOGLOBAL_SECTION

   
   This makes sure that no memory is allocated to XFC in Step 4, when the system reboots with VIOC.

2. Set the VCC_FLAGS system parameter to 1.
3. Run AUTOGEN to ensure that other system parameters allow for the new value. This is not essential, but it is advisable.
4. Reboot the system. VIOC is automatically loaded during startup instead of XFC, because VCC_FLAGS is 1.

If you forgot to remove the VCC$MIN_CACHE_SIZE entry from the reserved memory registry in Step 1, memory is allocated to XFC even though XFC is not loaded. Nothing can use this memory. If this happens, use the Sysman utility's RESERVED_MEMORY FREE command to release this memory:

$ RUN SYS$SYSTEM:SYSMAN SYSMAN> RESERVED_MEMORY FREE VCC$MIN_CACHE_SIZE /NOGLOBAL_SECTION

18.6.3 Controlling the Size of the Cache

The way that you control the size of VIOC depends on whether you have an OpenVMS Alpha or OpenVMS VAX system.

OpenVMS Alpha

On OpenVMS Alpha systems, the size of VIOC is fixed at system startup time. The cache can't shrink or grow. The value of the static system parameter VCC_MAXSIZE specifies the size of the cache in blocks. By default it is 6400 blocks (3.2MB).

To change the size of VIOC on an OpenVMS Alpha system, follow these steps:

1. Set the VCC_MAXSIZE system parameter to the required value.
2. Run AUTOGEN to ensure that other system parameters allow for the new value. This is not essential, but it is advisable.
3. Reboot the system to make the new value effective.

OpenVMS VAX

On OpenVMS VAX systems, you can use the static system parameter VCC_PTES to specify the maximum size of VIOC. This parameter specifies the size in pages. By default it is 2,000,000,000.

VIOC automatically shrinks and grows, depending on your I/O workload and how much spare memory is available on your system. As your I/O workload increases, the cache automatically grows, but never to more than the maximum size. And when your applications need memory, the cache automatically shrinks.

To change the maximum size of VIOC on an OpenVMS VAX system, follow these steps:

1. Set the VCC_MAXSIZE system parameter to the required value.
2. Run AUTOGEN to ensure that other system parameters allow for the new value. This is not essential, but it is advisable.
3. Reboot the system to make the new value effective.

Use the DCL command SHOW MEMORY/CACHE/FULL to display statistics about the virtual I/O cache, as shown in the following example:

$ SHOW MEMORY/CACHE/FULL System Memory Resources on 10-OCT-1994 18:36:12.79 Virtual I/O Cache Total Size (pages) (1) 2422 Read IO Count (6) 9577 Free Pages (2) 18 Read Hit Count (7) 5651 Pages in Use (3) 2404 Read Hit Rate (8) 59% Maximum Size (SPTEs) (4) 11432 Write IO Count (9) 2743 Files Retained (5) 99 IO Bypassing the Cache (10) 88

By default, virtual I/O caching is enabled. Use the following system parameters to enable or disable caching. Change the value of the parameters in MODPARAMS.DAT, as follows:

Parameter	Enabled	Disabled
VCC_FLAGS (Alpha)	1	0
VBN_CACHE_S (VAX)	1	0

Once you have updated MODPARAMS.DAT to change the value of the appropriate parameter, you must run AUTOGEN and reboot the node or nodes on which you have enabled or disabled caching. Caching is automatically enabled or disabled during system initialization. No further user action is required.

18.6.6 Determining If VIOC Is Enabled

SHOW MEMORY/CACHE indicates whether VIOC caching is on or off on a running system. (This is a lot easier than using SYSGEN.)

SYSGEN can be used to examine parameters before a system is booted. For example, you can check the system parameter VCC_FLAGS (on Alpha) or VBN_CACHE_S (on VAX) to see if virtual I/O caching is enabled by using SYSGEN, as shown in the following Alpha example:

$ RUN SYS$SYSTEM:SYSGEN SYSGEN> SHOW VCC_FLAGS

A value of 0 indicates that caching is disabled; the value 1 indicates caching is enabled.

18.6.7 Memory Allocation and VIOC

The memory allocated to caching is determined by the size of the free-page list. The size of the virtual I/O cache can grow if one of the following conditions is true:

• If the amount of available free memory is twice the value of FREEGOAL and if proactive memory reclamation is enabled for periodically waking processes.
• If the amount of available free memory is equal to the value of FREEGOAL and if proactive memory reclamation is enabled for long-waiting processes.
• If the amount of available free memory is greater than GROWLIM and if proactive memory reclamation is not enabled.

The cache size is also limited by the following:

• The number of system page table entries (SPTE) that are available. This number is a calculated value determined at boot time.
• The demands of the memory management subsystem. The memory management subsystem has a direct interface to cache so that, when necessary, it can demand that the cache return space to it.

How is memory reclaimed from the cache? The swapper can reclaim memory allocated to the virtual I/O cache by using first-level trimming. In addition, a heuristic primitive shrinks the cache returning memory in small increments.

18.6.8 Adjusting VIOC Size

The size of the virtual I/O cache is controlled by the system parameter VCC_MAXSIZE. The amount of memory specified by this parameter is statically allocated at system initialization and remains owned by the virtual I/O cache.

To increase or decrease the size of the cache, modify VCC_MAXSIZE and reboot the system.

18.6.9 VIOC and OpenVMS Cluster Configurations

The cache works on all supported configurations from single-node systems to large mixed-interconnect OpenVMS Cluster systems. The virtual I/O cache is nodal; that is, the cache is local to each OpenVMS Cluster member. Any base system can support virtual I/O caching; an OpenVMS Cluster license is not required to use the caching feature.

The lock manager controls cache coherency. The cache is flushed when a node leaves the OpenVMS Cluster. Files opened on two or more nodes with write access on one or more nodes are not cached.

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