Wednesday, September 16, 2020

RDBMS / ASM / Exadata - "Smart Rebalance" / Seems like the %15 free space rule (or %9 free space rule) is becoming history

Yesterday, I published a blog post about the %15 rule. I shared my thoughts on %15 free size rule, which states that, in order to be in the safe side in case of a cell/or disk failure in Exadata/ASM environments, we need to have some free space in the relevant diskgroups. This actually guarantees the rebalance, which should be done after a disk or cell failure, to be succesful. The rule states that, at least %15 of a diskgroup should be free. 

I found this magic number, or let's say this magic percentage (%15) a little interesting and felt the need to write a publish a blog post about it.

You can access that blog post via the url below;
http://ermanarslan.blogspot.com/2020/09/asm-grid-my-thougths-on-15-free-disk.html

Yestarday night, I was still curious and checking the documents to learn something new about this subject and I finally found the thing I was looking for. That was exactly what was expected. "Smart Rebalance"

Smart rebalance used with Oracle ASM which eliminates the need for free space for Grid Infrastructure 19c and Higher using high redundancy diskgroups.

In other words;  no need for free space!   If there is not enough space to rebalance at the time of failure, offline the disk! Upon replacement, efficiently repopulate it from partner disks automatically! 
This eliminates the need to reserve free space for rebalance when using high redundancy. It provides  seamless repair without the risk of out of space errors..

Currently there is no internal info about it, but you may visit the following the url to see  Exadata MAA slides.. Slide 58 introduces the smart rebalance and shows the dissaperance of %15 rule gradually :)


I will give you more information on this topic when I have a little more detail.
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2 comments :

  1. OnurSeptember 16, 2020 at 10:37 PM

    While checking error "ORA-15041" found Doc ID 1367078.1 .

    One line from doc says;
    "If any one disk is short of free_mb, then the error might be seen, even if there is sufficient free space in the whole diskgroup."

    Which means that ASM can allocate different sizes on different disks , you can have sufficient space in total but it doesn't mean that re-balance is guaranteed.

    Another line from doc;

    "This is a must that you have all the disks of same size, in a diskgroup. Starting 10.2, the total size of the disk is taken into consideration for allocations. So there will be imbalanced IO to disks. A future task would be to add/drop disks to have all the disks of same size."

    Above statement supports that %15 free space limit was there since ASM was not smart enough to allocate disks evenly. Until version 19c which has smart ASM

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