Starting with Grid Infrastructure/ASM 19.16 Oracle changed how the REQUIRED_MIRROR_FREE_MB is calculated and the impact is more than expected. Check below examples of the changes, and how this will impact you. This is valid for all GI/ASM starting with 19.16 and only for Exadata/ExaCC.
“amount of space that must be available in a disk group to restore full redundancy after the worst failure that can be tolerated by the disk group without adding additional storage. This requirement ensures that there are sufficient failure groups to restore redundancy”.
And (at Exadata environment until 19.16) is calculated based on the disk redundancy that you have. If you choose the HIGH, the raw size of two disks (the largest in your diskgroup) is reserved; at NORMAL, is the raw size of one disk. At Exadata, it differs from other environments because does not consider the whole failgroup failure and the way that extends are written/spread (more info below and in another post).
But for now, understand that the required size is what you need to reserve (as raw space) at your diskgroup to ensure protection in case of disk failure. And it is directly related to the USABLE_FILE_MB because the space that you can allocate at your diskgroup (USABLE_FILE_MB) comes from (FREE_MB- REQUIRED_MIRROR_FREE_MB)/redundancy factor (3 for HIGH, 2 for NORMAL). So, when you increase the REQUIRED_MIRROR_FREE_MB you reduce the USABLE_FILE_MB. I will explain more later.
The REPLACE DISK command was released with 12.1 and allow to do an online replacement for a failed disk. This command is important because it reduces the rebalance time doing just the SYNC phase. Comparing with normal disk replacement (DROP and ADD in the same command), the REPLACE just do mirror resync.
Basically, when the REPLACE command is called, the rebalance just copy/sync the data from the survivor disk (the partner disk from the mirror). It is faster since the previous way with drop/add execute a complete rebalance from all AU of the diskgroup, doing REBALANCE and SYNC phase.
The replace disk command is important for the SWAP disk process for Exadata (where you add the new 14TB disks) since it is faster to do the rebalance of the diskgroup.
Survive to disk failures it is crucial to avoid data corruption, but sometimes, even with redundancy at ASM, multiple failures can happen. Check in this post how to use the undocumented feature “mount restricted force for recovery” to resurrect diskgroup and lose less data when multiple failures occur.
Diskgroup redundancy is a key factor for ASM resilience, where you can survive to disk failures and still continue to run databases. I will not extend about ASM disk redundancy here, but usually, you can configure your diskgroup without redundancy (EXTERNAL), double redundancy (NORMAL), triple redundancy (HIGH), and even fourth redundancy (EXTEND for stretch clusters).
If you want to understand more about redundancy you have a lot of articles at MOS and on the internet that provide useful information. One good is this. The idea is simple, spread multiple copies in different disks. And can even be better if you group disks in the same failgroups, so, your data will have multiple copies in separate places.
As an example, this a key for Exadata, where every storage cell is one independent failgroup and you can survive to one entire cell failure (or double full, depending on the redundancy of your diskgroup) without data loss. The same idea can be applied at a “normal” environment, where you can create failgroup to disks attached to controller A, and another attached to controller B (so the failure of one storage controller does not affect all failgroups). At ASM, if you do not create failgroup, each disk is a different one in diskgroups that have redundancy enabled.
Recently I executed the upgrade of Oracle GI to 19c version, from 126.96.36.199 to 188.8.131.52 version. But one step that was not showed there was that, because of requirements, the GI was upgraded from 184.108.40.206 to 220.127.116.11. This upgrade is a just Release Update (RU) apply and opatchauto command.
But during this upgrade, from 18.2 to 18.6, I faced (more than one time – 5 to be precise) errors during the update because of the MGMTDB errors. I got these errors:
ORA-12514, TNS: Listener does not currently know of service requested in connect descriptor
CRS-10407: (:CLSCRED1079:)Credential domain does not exist.
Here I will show how to solve these errors, how to identify if everything was fine and if you can continue. Be careful that it is an example, always open a support SR to identify the source of the error.