Category: Oracle
2017-04-14 10:19:44
The unit of space allocation in ASM is AU, and Extent contains one or more AUs.
Before 11g, 1 Extent corresponds to 1 AU. Since 11.1.0, Oracle has introduced Variable Sized Extents. ASM will automatically determine the size of the Extent according to the size of the file. The specific algorithm is as follows:
1. If the AU is less than 4MB:
By introducing Variable Sized Extents, the memory of the SGA can be reduced and the performance can be improved.
Let's take a look at the Extent distribution in the real environment:
1. Small data files
The data file is only 790MB. According to the above allocation rules, the Extent=1*AU of the data file
The above information shows that there is indeed only one case of 1AU.
2. Larger data files
The data file is 24G. According to the above allocation rules, the Extent of the data file has two situations: 1AU and 4AU
The above information shows that the actual distribution is in line with the distribution rules.
Before 11g, 1 Extent corresponds to 1 AU. Since 11.1.0, Oracle has introduced Variable Sized Extents. ASM will automatically determine the size of the Extent according to the size of the file. The specific algorithm is as follows:
1. If the AU is less than 4MB:
- The first 20000 extents (0-19999, 20GB), extent size=1*AU
- After 20000 extents (20000-39999, 20GB-40GB), extent size=4*AU
- Further Extent (40000+, more than 40GB), Extent size=16*AU
By introducing Variable Sized Extents, the memory of the SGA can be reduced and the performance can be improved.
Let's take a look at the Extent distribution in the real environment:
1. Small data files
| FILE_NAME SIZE(MB) --------------------------------------------------------------------------- ---------- +DATA01/HERA/DATAFILE/system.258.940635333 790 |
The data file is only 790MB. According to the above allocation rules, the Extent=1*AU of the data file
| 09:31:28 nolog> select distinct size_kffxp from x$kffxp where number_kffxp = 258; SIZE_KFFXP ---------- 1 |
The above information shows that there is indeed only one case of 1AU.
2. Larger data files
| FILE_NAME SIZE(MB) --------------------------------------------------------------------------- ---------- +DATA01/HERA/DATAFILE/test.363.941274405 24576 |
The data file is 24G. According to the above allocation rules, the Extent of the data file has two situations: 1AU and 4AU
| 09:03:51 nolog> select distinct size_kffxp from x$kffxp where number_kffxp = 363; SIZE_KFFXP ---------- 1 4 |
The above information shows that the actual distribution is in line with the distribution rules.