Document revision date: 15 July 2002

On larger machines, XFC may be somewhat limited by the default size of the modified page list. In general, the modified page list can be considered a 1 to 1 correspondence data cache. XFC is a many to one cache; that is, in general, a cached page is accessed by many users. On large memory systems, AUTOGEN usually sets MPW_HILIMIT to a very large value. This may mean that there are not enough free pages for the memory management subsystem to give to XFC.

You can force XFC to have a minimum number of pages as specified in Setting a Minimum Size. You can also permanently allocate memory just for XFC, which prevents any overhead caused by the dynamic allocation and deallocation routines (similar in operation to VIOC). To do this, set system parameter VCC_MAX_CACHE to be equal to the memory reservation specified by VCC$MIN_CACHE_SIZE.

For example, if your system has 128GB of memory, you may find that by dedicating 8GB of memory to XFC, your cache hit rates and overall response time is consistently good. For example:

$ RUN SYS$SYSTEM:SYSGEN SYSGEN> USE CURRENT SYSGEN> SET VCC_MAX_CACHE 8000 SYSGEN> WRITE ACTIVE $ RUN SYS$SYSTEM:SYSMAN SYSMAN> RESERVED_MEMORY ADD VCC$MIN_CACHE_SIZE /SIZE=8000 /ALLOCATE - _SYSMAN> /NOGLOBAL_SECTION /NOZERO /NOPAGE_TABLE/RAD=0

18.5.2 Controlling the Maximum Cached I/O Size

The dynamic system parameter VCC_MAX_IO_SIZE controls the maximum size of I/O that can be cached by XFC. This parameter specifies the size in blocks. By default, it is 128.

Example

This example changes the maximum size of I/O that can be cached by XFC to 1,000 blocks:

$ RUN SYS$SYSTEM:SYSGEN SYSGEN> USE CURRENT SYSGEN> SET VCC_MAX_IO_SIZE 1000 SYSGEN> WRITE ACTIVE

This series of commands affects I/Os to volumes currently mounted on the local node, as well as to volumes mounted in the future. After you enter these commands, XFC does not cache I/Os that are bigger than 1,000 blocks. The SHOW MEMORY /CACHE /FULL command provides a histogram of I/O sizes that can provide guidelines for efficient parameter setting.

18.5.3 Disabling Caching for a File

To prevent XFC to from caching a particular file, such as a database file, set the caching attribute of the file to no caching.

The caching attribute of a file is the default caching option that is used by XFC when an application accesses the file without specifying which caching option it wants to use. The caching option can be either write-through caching or no caching.

If you want a file to be cached, set its caching attribute to write-through (the default). If you don't want a file to be cached, set its caching attribute to no caching.

Use...	To...
SET FILE /CACHING_ATTRIBUTE=keyword +	Set the caching attribute of a file or directory
DIRECTORY /CACHING_ATTRIBUTE or DIRECTORY /FULL	Show the caching attribute of a file or directory
DIRECTORY /SELECT=CACHING_ATTRIBUTE=(keyword[,...]) +	Show all the files and directories that have a particular caching attribute

XFC does not cache directories. The caching attribute of a directory controls only how the caching attribute is inherited by new files and subdirectories created in the directory:

• When you create a new directory or file, it inherits its caching attribute from its parent directory.
• When you create a new version of an existing file, the new file inherits its caching attribute from the highest version of the existing file.

Examples

• This example sets the caching attribute to no caching for all the files in and under the directory [SMITH.BORING].
  The first SET FILE command sets the attribute for the directory to make sure that all files and subdirectories subsequently created in it inherit the attribute. The second SET FILE command sets the attribute for all existing files and directories in and under the directory.

  $ SET FILE DISK$USERS:[SMITH]BORING.DIR;1 /CACHING_ATTRIBUTE=NO_CACHING $ SET FILE DISK$USERS:[SMITH.BORING...]*.*;* /CACHING_ATTRIBUTE=NO_CACHING

• This example uses the DIRECTORY command's /CACHING_ATTRIBUTE qualifier to show the caching attribute of MYFILE.TXT:

  $ DIRECTORY MYFILE.TXT /CACHING_ATTRIBUTE Directory DISK$USERS:[SMITH] MYFILE.TXT;1 Write-through Total of 1 file.

• This example shows all the files in the volume DISK$USERS that have a caching attribute of no caching.

  $ DIRECTORY DISK$USERS:[000000...]*.* /SELECT=CACHING_ATTRIBUTE=NO_CACHING

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$M_NOVCACHE.
(8) Files cached open	The number of open files currently being cached.
(9) Volumes in Full XFC mode	Reserved for Compaq. Should be 0.
(10) Volumes in No Caching mode	If caching is disabled on the local node or on another node in the OpenVMS Cluster, this is the number of volumes that are currently mounted on the local node. Otherwise it is zero.
(11) Maximum size	The maximum size that the cache could ever grow to.
(12) Minimum size	The minimum size that the cache could ever shrink to. This is controlled by the value of the VCC$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	The number of volumes being cached by XFC that are using the VCC caching protocol. As of OpenVMS Version 7.3, XFC uses only VCC caching protocol.
(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.

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