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In Oracle Database 10g Release 2, you can also manage ASM using a command-line tool, which gives you more flexibility than having to use SQL*Plus or the Database Control. To invoke the command-line administrative tool, called asmcmd, enter this command (after the ASM instance is started): $ asmcmd ASMCMD> The command-line tool has about a dozen commands you can use to manage ASM file systems, and it includes familiar UNIX/Linux commands such as du, which checks ASM disk usage. To get a complete list of commands, type help at the command prompt (ASMCMD>). By typing help followed by a command, you can get details about that command.

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An ASM disk group is a collection of disks analogous to the logical volumes that an LVM creates from the underlying physical disks. This means that you have to manage the underlying disks indirectly by managing the disk group. If you have large numbers of disks, you can group them into a small number of easily managed disk groups, and if you add storage to your ASM system, you simply add disks to an ASM disk group. This is good news, because if your database grows quickly, the total storage space increases, but the number of disk groups remains the same.

Two major reasons for using ASM file management are the additional performance and protection, and the decreased management overhead. Of course, these are the same advantages third-party vendors claim for their LVM tools, but the major advantage of ASM is that you as an Oracle DBA can do most of the disk management using ASM. There s no need for you to be an expert in file systems, RAID, or logical volumes to use ASM; all you need is an understanding of ASM s disk-management system and Oracle s processes for accessing database files spread over the ASM disks. ASM gives you performance and redundancy through striping and mirroring, so let s look at these two features.

Note The OEM Database Control is the best way to administer the ASM instance, once you create it. Refer to the Oracle Database 10g Release 2 (10.2) manual Oracle Database 2 Day DBA for details about using Database Control to manage disk groups, as well as all other aspects of an ASM instance.

If you want to find out what concrete kind of request a WebRequest^ variable refers to, you can use the dynamic_cast operator, as follows: WebRequest^ req = GetWebRequestFromSomeWhere(); if (dynamic_cast<FtpWebRequest^>(req) != nullptr) Console::WriteLine("The request was an FTP request");

When using the following command, there is a very subtle syntax difference that you may use but will have quite different results:

ASM systems store your database files on ASM disks. The manner in which you place your database files on ASM disks plays a critical role in the resulting performance. For optimal I/O performance, ASM stripes its files across every disk that is part of its disk group. This means that all the disks in a disk group must be of the same type and performance capacity. ASM offers two types of striping, with the choice depending on the type of database file. Coarse striping uses a stripe size of 1MB, and you can use coarse striping for every file in your database, except for the control files, online redo log files, and flashback files. Fine striping uses a stripe size of 128KB. You can use fine striping for control files, online redo log files, and flashback files.

Disk mirroring gives us data redundancy. This means that, should you lose a disk, you can use the mirror disk to continue operations. This process is not like an OS-level mirroring scheme, but they both provide redundancy for your database. The difference is that OS-based LVMs mirror entire disks, whereas ASM mirrors extents. This means that when ASM allocates an extent (the primary extent, in contrast to a mirrored extent), it also allocates a mirror copy to one of the disks in the same disk group. Therefore, a disk can have mirror extents on any number of disks in the group. When a disk in a group fails, ASM rebuilds the failed disk using the mirrored extents from other disks in the group. When ASM reconstructs a failed disk, the storage system takes a small performance hit, because ASM requires some extra I/O to reconstruct the failed device.

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