Oracle Automatic Storage Management - Concepts

  • Oracle ASM is a volume manager and a file system for Oracle database files.
  • It supports single-instance and Oracle RAC configurations.
  • Oracle ASM also supports a general purpose file system that can store application files and oracle database binaries.
  • It provides an alternative to conventional volume managers, file systems and raw devices.

  • Oracle ASM distributes I/O load across all available resource to optimize performance.
  • In this way, it removes the need for manual I/O tuning (spreading out the database files avoids hotspots).
  • Oracle ASM allows the DBA to define a pool of storage (disk groups).
  • The Oracle kernel manages the file naming and placement of the database files on the storage pool.
Disk groups
  • Oracle ASM store data files on disk groups.
  • A disk group is a collection of disks managed as a unit by Oracle ASM.
  • Oracle ASM disks can be defined on:
    • A disk partition: Entire disk or a section of disk that does not include the partition table (or it will be overwritten).
    • A Disk from a storage array (RAID): RAID present disks as Logical Unit Numbers (LUNs).
    • A logical volume.
    • A Network-attached file (NFS): Including files provided through Oracle Direct NFS (dNFS).Whole disks, partitions and LUNs can also be mounted by ASM through NFS.
  • Load balance: Oracle ASM spreads the files proportionally across all of the disks in the disk group, so the disks within a disk group should be in different physical drives.

Disks can be added or removed "on the fly" to and from disk groups.
After you add a disk, Oracle ASM performs rebalancing.
Data is redistributed to ensure that every file is evenly spread across all of the disks.

  • Disks can be added or removed from a disk group while the database is accessing files on that disk group (without downtime).
  • Oracle ASM redistributes contents automatically
  • Oracle ASM uses Oracle Managed Files (OMF).

  • Any Oracle ASM file is completely contained within a single disk group.
  • However, a disk group might contain files belonging to several databases.
  • A single database can use files from multiple disk groups.
Mirroring and Failure groups
  • Disk groups can be configured with varying redundancy levels.
  • For each disk in a disk group, you need to specify a failure group to which the disk will belong.
  • A failure group is a subset of the disks in a disk group, which could fail at the same time because they share hardware
  • Failure groups are used to store mirror copies of data.
  • In a normal redundancy file, Oracle ASM allocates a primary copy and a secondary copy in disks belonging to different failure groups.
  • Each copy is on a disk in a different failure group so that the simultaneous failure of all disks in a failure group does not result in data loss.
  • A normal redundancy disk group must contain at least two failure groups.
  • Splitting the various disks in a disk group across failure groups allows Oracle ASM to implment file mirroring.
  • Oracle ASM implements mirroring by allocating file and file copies to different failure groups.
  • If you do not explicitly identify failure groups, Oracle allocates each disk in a disk group to its own failure group.

Oracle ASM implements one of three redundancy levels:
  • External redundancy:
    • No ASM mirroring. Useful when the disk group contain RAID devices
  • Normal redundancy
    • Oracle ASM implements 2-way mirroring by default.
    • At least 2 failure groups are needed. Minimum of two disks in group.
  • High redundancy
    • Oracle ASM implements 3-way mirroring: Minimum of 3 disks in group

(a) Diskgr1 below implements 2-way mirroring.
Each disk (dasm-d1, dasm-d2) is assigned to its own failure group.
SQL> Create diskgroup diskgr1 NORMAL redundancy
  2  FAILGROUP controller1 DISK
  4     '/devices/diska1' NAME dasm-d1,
  3  FAILGROUP controller2 DISK
  5     '/devices/diskb1' NAME dasm-d1
  6  ATTRIBUTE 'au_size'='4M';

  • An Oracle ASM disk is divided into allocation units (AU).
  • Files within an ASM disk consist of one or more allocation units.
  • Each ASM file has one or more extents.
  • Extent size is not fixed: starting with one allocation unit, extent size increases as total file size increases.

Oracle ASM Instance

Oracle ASM metadata:
  • disks belonging to a disk group
  • space available in a disk group
  • names of files in a disk group
  • location of disk group data extents
  • redo log for changes in metadata blocks
  • Oracle ADVM (ASM Dynamic volume Manager) volume information
  • With Oracle ASM an ASM instance besides the database instance needs to be configured on the server.
  • An Oracle ASM instance has an SGA and background processes, but is usually much smaller than a database instance.
  • It has minimal (how much?) performance effect on a server.
  • Oracle ASM Instances are responsible for mounting the disk groups so that ASM files are available for DB instances.
  • Oracle ASM instances DO NOT mount databases.
  • They only manage the metadata of the disk group and provide file layout information to the database instances.

ASM Instance on Clusetered configurations:
  • One Oracle ASM instance in each cluster node.
  • All database instances in a node share the same ASM instance
  • In a Oracle RAC environment, the ASM and database instances on the surviving nodes automatically recover from an ASM Instance failure on a node.

TCP/IP Networking (I)

TCP/IP Architecture
  • TCP/IP protocol has a four-layer structure linking an application to the physical network.
  • Each layer has its own independent data structures.
  • Conceptually, each layer is speaking directly to its counterpart on the other machine. In this sense, it is ignorant of what goes one after the data is sent.
  • For example, in the Application layer, a NFS Client talks to a NFS Server and knows only the details of the NFS protocol they both use.
  • As data packets are transported from the application to the physical network, each layer adds some control information in the form of a header.
  • Once the packet reaches its destination in the physical network, each layer reads and removes its corresponding header before passing the package up in the stack until it is received by the application.

  • This layer contains all application protocols (often providing user services) that use the Transport layer.
  • Examples of application protocols include FTP, HTTP, DNS, NFS, SMTP, Telnet
  • To send data, the application calls up a Transport layer protocol, such as TCP.
  • Application Layer protocols usually treat transport and lower layer protocols as "black boxes." In this sense, they assume a stable network connection exist across which to communicate.

  • TCP and UDP are the most importan protocols in this layer, delivering data between application and internet layers.
  • TCP provides reliable data delivery service with error detection and error correction. It delivers data received from IP to the correct application (identified by a port number).
  • UPD provides a connectionless delivery service.
  • When called by an application, TCP wraps the data into a TCP packet.
  • A TCP packet (also called TCP segment) contains a TCP header followed by the application data (including header).
  • TCP then hands the packet to IP.
  • TCP keeps track of what data belongs to what process.
  • It is also responsible for ensuring that the packets are delivered with the correct contents and put in the right order before handing them off to the receiving application.

  • The layer above the Network Access layer, and it provides the packet delivery service on which TCP/IP networks are built.
  • It provides a routing mechanism allowing for packets to be transmitted across one or more different networks.
  • The Internet Protocol (IP) runs in this layer and provides a way to transport datagrams across the network.
  • It is a connectionless protocol and does not provide error control, relying on protocols in the other layers to provide error detection and recovery.
  • Source and destination may be in the same or different networks.
  • The IP protocol performs the functions of (a) host addressing and identification, and (b) packet routing (transporting packets from source to destination).
  • After receiving a TCP packet, IP wraps it up and prepends an IP header, creating an IP datagram.
  • Moving the data down the stack, IP hands it off to the hardware driver, that runs in the Network Access Layer.

  • The IP layer has to figure out how to send the packet.
  • Destination on a different physical network ?
    • Then IP needs to find and send it to the appropriate gateway.
  • Destination on the local ethernet network ?
    • IP uses the Address Resolution Protocol (ARP) to determine what Ethernet card's MAC address is associated with the datagram IP address.
  • How does it work?
    • ARP broadcasts an ARP packet across the entire network asking which MAC address belongs to a particular IP address.
    • Although every machines gets this broadcast, only the one out there that matches will respond. This is then stored by the IP layer in its internal ARP table.

You can look at the ARP table at any time by running the command:
jdoe@quark:~$ arp -a
home ( at 98:0:bd:bd:8c:d2 [ether] on eth0

  • Protocols in this layer are designed to move packets (IP datagrams) between the internet layer interface of two different hosts on the same physical link.
  • The actual process of moving packets at this level is usually controlled by device drivers of the network cards, which must know the details of the underlying network in order to format the data appropriately.
  • At this level IP addresses are translated to physical addresses used by the network cards (i.e. Media Access Control (MAC) addresses)
  • The network access layer (also called link layer) can be represented by different kinds of physical connections: Ethernet, token-ring, fiber-optics, ISDN, RS-232, etc.

Network Interfaces
  • TCP/IP defines an abstract interface for hardware access.
  • The interface, offering a set of operations that is used to access all types of hardware, hides the implementation details of operations necessary to access each particular equipment. Each vendor is responsible for provinding a driver that translates the commands of the TCP/IP interface to those of the particular piece of hardware.
  • Each networking device has a corresponding interface in the kernel.
  • When configured, each physical device is assigned an interface name.
  • Each interface must also be assigned an IP address. Some interface names include:
    • Ethernet interfaces: eth0, eth1
    • PPP interfaces: ppp0, ppp1
    • FDDI interfaces: fddi0, fddi1
  • A computer having more than one logical or physical network interface is usually called a Multihomed host.

  • An Ethernet network works like a bus system, where a host may send packets (or frames) of up to 1,500 bytes to another host on the same Ethernet.
  • Hosts are identified by a six-byte address hardcoded into the firmware of its Ethernet network interface card (NIC).
  • Ethernet addresses are usually written as a sequence of two-digit hex numbers separated
    by colons, as in aa:bb:cc:dd:ee:ff.

Bautts, Tony, Terry Dawson and Gregor Prudy. 2005. Linux Network Administratos Guide
Hunt, Craig. 2002. TCP/IP Network Administration