Managing Capacity in Disk Groups
You must have sufficient spare capacity in each disk group to handle the largest failure
that you are willing to tolerate. After one or more disks fail, the process of restoring
redundancy for all data requires space from the surviving disks in the disk group. If
not enough space remains, some files may end up with reduced redundancy.
Reduced redundancy means that one or more extents in the file are not mirrored at the
expected level. For example, a reduced redundancy file in a high redundancy disk
group has at least one file extent with two or fewer total copies of the extent instead of
three. In the case of unprotected files, data extents could be missing altogether. Other
causes of reduced redundancy files are disks running out of space or an insufficient
number of failure groups. The V$ASM_FILE column REDUNDANCY_LOWERED indicates
a file with reduced redundancy.
The following guidelines help ensure that you have sufficient space to restore full
redundancy for all disk group data after the failure of one or more disks.
■ In a normal redundancy disk group, it is best to have enough free space in your
disk group to tolerate the loss of all disks in one failure group. The amount of free
space should be equivalent to the size of the largest failure group.
■ In a high redundancy disk group, it is best to have enough free space to cope with
the loss of all disks in two failure groups. The amount of free space should be
equivalent to the sum of the sizes of the two largest failure groups.
The V$ASM_DISKGROUP view contains columns that help you manage capacity:
■ REQUIRED_MIRROR_FREE_MB indicates the amount of space that must be
available in the disk group to restore full redundancy after the worst failure that
can be tolerated by the disk group.
■ USABLE_FILE_MB indicates the amount of free space, adjusted for mirroring, that
is available for new files.
Administering Automatic Storage Management Disk Groups
12-18 Oracle Database Administrator’s Guide
USABLE_FILE_MB is computed by subtracting REQUIRED_MIRROR_FREE_MB from
total free space in the disk group and then adjusting for mirroring. For example, in a
normal redundancy disk group, where by default mirrored files take up disk space
equal to twice their size, if 4 GB of actual usable file space remains, USABLE_FILE_MB
equals roughly 2 GB. You can then add up to a 2 GB file.
The following query shows capacity metrics for a normal redundancy disk group that
consists of six 1 GB (1024 MB) disks, each in its own failure group:
select name, type, total_mb, free_mb, required_mirror_free_mb,
usable_file_mb from v$asm_diskgroup;
NAME TYPE TOTAL_MB FREE_MB REQUIRED_MIRROR_FREE_MB USABLE_FILE_MB
------------ ------ ---------- ---------- ----------------------- --------------
DISKGROUP1 NORMAL 6144 3768 1024 1372
The REQUIRED_MIRROR_FREE_MB column shows that 1 GB of extra capacity must be
available to restore full redundancy after one or more disks fail. Note that the first
three numeric columns in the query results are raw numbers. That is, they do not take
redundancy into account. Only the last column is adjusted for normal redundancy.
Notice that:
FREE_MB - REQUIRED_MIRROR_FREE_MB = 2 * USABLE_FILE_MB
or
3768 - 1024 = 2 * 1372 = 2744
Negative Values of USABLE_FILE_MB Due to the relationship between FREE_MB,
REQUIRED_MIRROR_FREE_MB, and USABLE_FILE_MB, USABLE_FILE_MB can go
negative. Although this is not necessarily a critical situation, it does mean that:
■ Depending on the value of FREE_MB, you may not be able to create new files.
■ The next failure may result in files with reduced redundancy.
If USABLE_FILE_MB becomes negative, it is strongly recommended that you add
more space to the disk group as soon as possible.
Scalability
ASM imposes the following limits:
■ 63 disk groups in a storage system
■ 10,000 ASM disks in a storage system
■ 4 petabyte maximum storage for each ASM disk
■ 40 exabyte maximum storage for each storage system
■ 1 million files for each disk group
■ Maximum files sizes as shown in the following table:
Disk Group Type Maximum File Size
External redundancy 35 TB
Normal redundancy 5.8 TB
High redundancy 3.9 TB

 

 

 

V$ASM_DISKGROUP

In an Automatic Storage Management instance, V$ASM_DISKGROUP displays one row for every disk group discovered by the Automatic Storage Management instance. In a database instance, V$ASM_DISKGROUP displays one row for every Automatic Storage Management disk group mounted by the local Automatic Storage Management instance.