ORACLEAUTOMATICSTORAGEMANAGEMENT-Chapter 10 ASM built-in data structure (2)

Free SpaceTable (FST)

FST indicates the free space contained in ATB. When the disk is allocated, ASM generates a disk FST. This allows ASM to skip the space occupied by ATBs. FST contains entries for each ATB. Each entry describes the total number of extents sizes and the number of idle extents. FST aims to optimize the allocation operation. In fact, FST is located in AllocationTable. FST is located in the second block (block 1) of each ).

PartnershipStatus Table (PST)

PST tracks the relationship between the disk qualification and the disk. ASM generates PST to determine whether a group of online disk attached disk meets the requirements. Each disk in the disk group retains the second AU (AU 1) for PST. However, at any time, a disk group only has a certain number of active PSTs. The number of PST depends on the disk Group Redundancy and the number of Failure groups ). The external redundant disk group has a PST. If there are at least three valid groups, the Normal-redundancy disk group contains three PSTs. However, each failure group of the normal-redundancy disk group has a PST. The High-redundancy disk group has five PST if there are enough failure groups. However, each failure group in the high-redundancy disk group has a PST.

PST is the shadow page used to ensure that the active PST on the disk is always the same. PST spans multiple blocks in a timely manner. AU 1 meets the need to write two PST copies. One copy is an active copy, and other copies are used when PST is updated. Once PST is updated, the previous Shadow becomes an active PST, and another copy becomes a shadow for the next update. RDBMS uses similar technologies to manage control files.

The first part of PST is about PST:

N Versionnumber

N Timestamp

N PSTsize (number of disks)

N Numberof PST copies

N Listof disk containing the PST

N Valueof compatible. asm (if COMPATIBLE. ASM >=11.1)

ASM stores COMPATIBLE. ASM in PST to reduce the storage quantity. to modify this information, submit it to the member. In this chapter, ASM also updates the COMPATIBLE. ASM values of all disk headers to prevent 10g software from attaching incompatible disk groups, while oracle database 11G uses the values in PAT.

PST contains a set of entries that describe each disk in the disk group. For each disk, PST tracks the following content:

N disk status (for example, online or offline)

N member count

N member list

The last block of PST stores the heartbeat of the disk group. ASM uses this block to ensure that the disk groups are not mounted at the same time in different clusters. Like RAC, ASM is protected by the Cluster Synchronization Service (CSS) to avoid split-brain in the Cluster. PST heartbeat is an additional mechanism to prevent split-brain on the cluster. This is like the heartbeat of RDBMS write control files to avoid two instances in different clusters mount a database at the same time.

Generally, a disk group can be attached only when it can access a certain number of PST. The Normal-redundancy disk group requires two invalid disk groups. The high-redundancy disk group requires five invalid disk groups and requires fewer PST to mount the disk group.

Virtual metadata

The virtual metadata is stored in the ASM file. Directory is the source data file accessed by the ASM instance in exclusive mode. The directory file number starts at 1. Not only can an RDBMS instance register and save the ASM file, but also can an ASM instance. The registration number is at least 255. The file number is not mentioned in this section to be reserved for future use. V $ asm_file does not display the metadata directory or registration.

For other ASM files, virtual metadata files are mirrored by the disk Group Redundancy type. ASM does not provide image protection for the external redundancy disk group. The virtual metadata provides three images in the normal-and high-redundancy disk groups. The following data structure constitutes virtual metadata:

N File Directory

N Disk Directory

N Active Change Directory (ACD)

N Continuing Operations Directory (DOD)

N Template Directory

N Alias Directory

N Attribute Directory

N Staleness Directory

N Staleness Registry

File Directory

File Directory contains all metadata related to the ASM File. The directory contains entries in each ASM file and these entries are indexed using numerical values. In each disk group, the file number in the file directory is 1 (F1 ).

File Directory contains the following fields:

N incarnation number

N File size

N File block size

N File type

N Redundancy (none, normal, or high)

N Striping (coarse or fine-grained)

N File creation time

N File modification time (the last time the File was opened or written)

N File layout

In ASM, if a file is deleted and the file number of the file is reused to create a new file, the file avatar is unique and different from the previous one. The Avatar number is generated based on the timestamp when the data file is created. Ensure that the Avatar number is unique when the ASM file name is the same.

The block size of the ASM file does not depend on the size of the ASM metadata block. All ASM meta directories use 4 K block size. The RDBMS data file can be set to 2 K, 4 K, 8 K, 16 K, or 32 K in the created tablespace. The redo log file of RDBMS is generally set to bytes. ASM and track the size of logical file blocks (as seen in data blocks). It also tracks the physical space used in the disk group and stores it in the audit file for future reference. RDBMS also provides the file type during file creation. For files that are not explicitly created by RDBMS instances (for example, created using the XMLDB ftp command or the cp command of ASMCMD), ASM checks the file header to determine the file type when creating the file.

The meaning of create time is obvious. However, filemodification time is not updated every time a data file is written. On the contrary, file modification time updates the timestamp when the file needs to be written but the file is opened. This means that the modification time will be modified, that is, no data is written, and the stored modification time may be earlier than the last modification of the file content. Similarly, to reduce the contention of File Directory blocks in the buffer cache in the ASM instance, modificationtime is truncated on time every hour. Therefore, if several instances in the cluster open files in a very short time, only the first instance accesses the file and updates the file modification time.

File distribution information is grouped into a series of extentpointers. Extent pointer specifies the disk number and AU Number of the extent. A FileDirectory entry contains the first 60 extentpointers of each file, sometimes called direct extent (direct extension ). The rest of FileDirectory space stores indirect extent (non-direct extension ). Indirect extent is another virtual metadata extent including the extent pointer of ASM. Each indirect extent is an AU. Each File Directory entry can be saved to 300 indirect extent.

The concept of indirect pointer is available in traditional file systems, such as Unix BSD file systems. The file layout of the presentation is also seen as extent map. Details about how striping and redundancy affect the file layout will be discussed in chapter 11th, "ASM Operations ".

Figure 10-1 shows a large enough file to store indirect extent. Note that indirect extent is metadata extended beyond File Directory. In this example, the disk group contains 10 ASM disks (D0 to D9 ).

Figure 10-2 shows the relationship between extentpointer and Allocat Table. In this example, file 256 has 5 extent (0 to 4) and file 257 has 4 extent (0 to 3 ). The disk group has three Disks (0 to 2 ). The figure shows only the files involved in the example. This is a small part of Allocation Table.