Data Blocks（摘自11gR2 concept）

Overview of Data Blocks

Oracle Database manages the logical storage space in the data files of a database in units called data blocks, also called Oracle blocks or pages. A data block is the minimum unit of database I/O.

Data Blocks and Operating System Blocks

At the physical level, database data is stored in disk files made up of operating system blocks. An operating system block is the minimum unit of data that the operating system can read or write. In contrast, an Oracle block is a logical storage structure whose size and structure are not known to the operating system.

shows that operating system blocks may differ in size from data blocks. The database requests data in multiples of data blocks, not operating system blocks.

Figure 12-5 Data Blocks and Operating System Blocks

When the database requests a data block, the operating system translates this operation into a requests for data in permanent storage. The logical separation of data blocks from operating system blocks has the following implications:

• Applications do not need to determine the physical addresses of data on disk.

• Database data can be striped or mirrored on multiple physical disks.

Database Block Size

Every database has a database block size. The DB_BLOCK_SIZE initialization parameter sets the data block size for a database when it is created. The size is set for the SYSTEM and SYSAUX tablespaces and is the default for all other tablespaces. The database block size cannot be changed except by re-creating the database.

If DB_BLOCK_SIZE is not set, then the default data block size is operating system-specific. The standard data block size for a database is 4 KB or 8 KB. If the size differs for data blocks and operating system blocks, then the data block size must be a multiple of the operating system block size.

原文：

结合linux文件系统
[root@a ~]# dumpe2fs /dev/mapper/vg_a-LogVol01 | grep size
dumpe2fs 1.41.12 (17-May-2010)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Block size:               4096
Fragment size:            4096
Flex block group size:    16
Inode size:               256
Required extra isize:     28
Desired extra isize:      28
Journal size:             128M
[root@a ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_a-LogVol01
                       15G   12G  2.0G  86% /
tmpfs                 372M   72K  372M   1% /dev/shm
/dev/sda1             194M   29M  156M  16% /boot
[root@a ~]#