SESSIONS

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Property	Description
Parameter type	Integer
Default value	Derived: (1.1 * PROCESSES) + 5
Modifiable	No
Range of values	1 to 231
Basic	Yes

SESSIONS specifies the maximum number of sessions that can be created in the system. Because every login requires a session, this parameter effectively determines the maximum number of concurrent users in the system. You should always set this parameter explicitly to a value equivalent to your estimate of the maximum number of concurrent users, plus the number of background processes, plus approximately 10% for recursive sessions.

Oracle uses the default value of this parameter as its minimum. Values between 1 and the default do not trigger errors, but Oracle ignores them and uses the default instead.

The default values of the ENQUEUE_RESOURCES and TRANSACTIONS parameters are derived from SESSIONS. Therefore, if you increase the value of SESSIONS, you should consider whether to adjust the values of ENQUEUE_RESOURCES and TRANSACTIONS as well. (Note that ENQUEUE_RESOURCES is obsolete as of Oracle Database 10g release 2 (10.2).)

In a shared server environment, the value of PROCESSES can be quite small. Therefore, Oracle recommends that you adjust the value of SESSIONS to approximately 1.1 * total number of connections.

Oracle initializatoin parameters

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V$TRANSACTION_ENQUEUE

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V$TRANSACTION_ENQUEUE displays locks owned by transaction state objects.

Column	Datatype	Description
ADDR	RAW(4 | 8)	Address of lock state object
KADDR	RAW(4 | 8)	Address of lock
SID	NUMBER	Identifier for session holding or acquiring the lock
TYPE	VARCHAR2(2)	Type of lock. TX indicates transaction enqueue.
ID1	NUMBER	Lock identifier #1 (depends on type)
ID2	NUMBER	Lock identifier #2 (depends on type)
LMODE	NUMBER	Lock mode in which the session holds the lock: 0 - none 1 - null (NULL) 2 - row-S (SS) 3 - row-X (SX) 4 - share (S) 5 - S/Row-X (SSX) 6 - exclusive (X)
REQUEST	NUMBER	Lock mode in which the process requests the lock: 0 - none 1 - null (NULL) 2 - row-S (SS) 3 - row-X (SX) 4 - share (S) 5 - S/Row-X (SSX) 6 - exclusive (X)
CTIME	NUMBER	Time since current mode was granted
BLOCK	NUMBER	The lock is blocking another lock

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Last updated: 2009-11-12 Thursday

How to read Unix file /etc/group

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/etc/group is a text file which defines the groups to which users belong under Linux and UNIX operating system. Under Unix / Linux multiple users can be categorized into groups. Unix file system permissions are organized into three classes, user, group, and others. The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks, printers, and other peripherals. This method, amongst others, also enables the superuser to delegate some administrative tasks to normal users.
It stores group information or defines the user groups i.e. it defines the groups to which users belong. There is one entry per line, and each line has four parts separated by a colon (:)

1. group_name: It is the name of group. If you run ls -l command, you will see this name printed in the group field.
2. Password: Generally password is not used, hence it is empty/blank. It can store encrypted password. This is useful to implement privileged groups.
3. Group ID (GID): Each user must be assigned a group ID. You can see this number in your /etc/passwd file.
4. Group List: It is a list of user names of users who are members of the group. The user names must be separated by commas.

More about a user groups
Users on Linux systems are assigned to one or more groups for following reasons:

• To share files or other resource with a small number of users
• Ease of user management
• Ease of user monitoring
• Group membership is perfect solution for large Linux (UNIX) installation.
• Group membership gives you or your user special access to files and directories or devices which are permitted to that group

How to read UNIX file /etc/passwd

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File /etc/passwd in UNIX contains one entry per line for each user (or user account) of the system.  There are seven fields separated by a colon (:) symbol:

1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
4. Group ID (GID): The primary group ID (stored in /etc/group file)
5. User ID Info: The comment field. It allows you to add extra information about the users such as user's full name, phone number etc. This field use by finger command.
6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /.
7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.

/etc/passwd is only used for local users only. To see list of all users, enter:
$ cat /etc/passwd
To search for a username called tom, enter:
$ grep tom /etc/passwd

/etc/passwd file permission

The permission on the /etc/passwd file should be read only to users (-rw-r--r--) and the owner must be root:
$ ls -l /etc/passwd
Output:

-rw-r--r-- 1 root root 2659 Sep 17 01:46 /etc/passwd

Your password is stored in /etc/shadow file

Your encrpted password is not stored in /etc/passwd file. It is stored in /etc/shadow file. Almost, all modern Linux / UNIX line operating systems use some sort of the shadow password suite, where /etc/passwd has asterisks (*) instead of encrypted passwords, and the encrypted passwords are in /etc/shadow which is readable by the superuser only.

V$DISPATCHER_CONFIG

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V$DISPATCHER_CONFIG displays information about the dispatcher configurations and their attributes.

Column	Datatype	Description
CONF_INDX	NUMBER	Zero-based index of the DISPATCHERS configuration
NETWORK	VARCHAR2(128)	Network protocol or listening address of dispatchers (may be truncated)
DISPATCHERS	NUMBER	Number of dispatchers to maintain for the configuration
CONNECTIONS	NUMBER	Maximum number of concurrent connections per dispatcher
SESSIONS	NUMBER	Maximum number of concurrent sessions per dispatcher
POOL	VARCHAR2(4)	Indicates whether Connection Pooling is on: IN OUT BOTH OFF
TICKS	NUMBER	Tick size (in seconds) for Connection Pooling timeout
INBD_TIMOUT	NUMBER	Timeout duration (in ticks) for pooled inbound connections
OUTBD_TIMOUT	NUMBER	Timeout duration (in ticks) for pooled outbound connections
MULTIPLEX	VARCHAR2(4)	Indicates whether Session Multiplexing is on: IN OUT BOTH OFF
LISTENER	VARCHAR2(1200)	Listeners to register dispatchers with (may be truncated)
SERVICE	VARCHAR2(128)	Service names supported (may be truncated)

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Last updated: 2009-12-22 Tuesday

V$ENCRYPTION_WALLET

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V$ENCRYPTION_WALLET displays information on the status of the wallet and the wallet location for transparent data encryption.

Column	Datatype	Description
WRL_TYPE	VARCHAR2(20)	Type of the wallet resource locator (for example, FILE)
WRL_PARAMETER	VARCHAR2(4000)	Parameter of the wallet resource locator (for example, absolute filename if WRL_TYPE = FILE)
STATUS	VARCHAR2(9)	Status of the wallet: OPEN CLOSED UNDEFINED

Note:

1.       Is there any view to tell if wallet is opened or not? Use V$ENCRYPTION_WALLET, which is a feature available in 10.2.0.4 and 11g database versions.

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Last updated: 2009-12-22 Tuesday

V$FILE_HISTOGRAM

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V$FILE_HISTOGRAM displays a histogram of all synchronous single block reads on a per-file basis (for data files). The histogram has buckets of time intervals from < 1 ms, < 2 ms, < 4 ms, < 8 ms, ... < 2²¹ ms, < 2²² ms, and >= 2²² ms.

The histogram will not be filled unless the STATISTICS_LEVEL initialization parameter is set to ALL.

Column	Datatype	Description
FILE#	NUMBER	File number
SINGLEBLKRDTIM_MILLI	NUMBER	Amount of time the bucket represents (in milliseconds). If the duration = num, then this column represents waits of duration < num that are not included in any smaller bucket.
SINGLEBLKRDS	NUMBER	Number of waits of the duration belonging to the bucket of the histogram

Note:

The v$filestat view displays statistics per data file. This information can be skewed due to a couple of very large I/O waits, thus these waits might not be indicative of the system as a whole.

V$FILE_HISTOGRAM displays a histogram of all single block reads on a per-file basis. The histogram has buckets of time intervals from < 1 ms, < 2 ms, < 4 ms, < 8 ms, ... < 2^21 ms, < 2^22 ms, >= 2^22 ms. The histogram can be used to determine if the bottleneck is a regular or a unique problem. The frequency of each wait event versus the length of the wait events would be seen using the information in the v$file_histogram view.

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Last updated: 2009-12-22 Tuesday

V$DB_PIPES

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======================================================================

V$DB_PIPES displays the pipes that are currently represented in the shared pool for this instance.

Column	Datatype	Description
OWNERID	NUMBER	Owner ID of the owner (if this is a private pipe), else NULL
NAME	VARCHAR2(1000)	Name of the pipe (for example, SCOTT.PIPE)
TYPE	VARCHAR2(7)	Type of the pipe: PUBLIC PRIVATE
PIPE_SIZE	NUMBER	Amount of memory the pipe uses

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Last updated: 2009-12-22 Tuesday