Uwe Hesse

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Homepage: http://uhesse.com

Brief introduction to ASM mirroring

Automatic Storage Management (ASM) is becoming the standard for good reasons. Still, the way it mirrors remains a mystery for many customers I encounter, so I decided to cover it briefly here.

ASM Basics: What does normal redundancy mean at all?

ASM normal redundancy

It means that every stripe is mirrored once. There is a primary on one disk and a mirror on another disk. All stripes are spread across all disks. High redundancy would mean that every primary stripe has two mirrors, each on another disk. Obviously, the mirroring reduces the usable capacity: It’s one half of the raw capacity for normal redundancy and one third for high redundancy. The normal redundancy as on the picture safeguards against the loss of any one disk.

ASM Basics: Spare capacity

ASM spare capacity usage

When disks are lost, ASM tries to re-establish redundancy again. Instead of using spare disks, it uses spare capacity. If enough free space in the diskgroup is left (worth the capacity of one disk) that works as on the picture above.

ASM 11g New Feature: DISK_REPAIR_TIME

What if the disk from the picture above is only temporarily offline and comes back online after a short while? These transient failures have been an issue in 10g, because the disk got immediately dropped, followed by a rebalancing to re-establish redundancy. Afterwards an administrator needed to add the disk back to the diskgroup which causes again a rebalancing. To address these transient failures, Fast Mirror Resync was introduced:

disk_repair_time

No administrator action required if the disk comes back before DISK_REPAIR_TIME (default is 3.6 hours) is over. If you don’t like that, setting DISK_REPAIR_TIME=0 brings back the 10g behavior.

ASM 12c New Feature: FAILGROUP_REPAIR_TIME

If you do not specify failure groups explicitly, each ASM disk is its own failgroup. Failgroups are the entities across which mirroring is done. In other words: A mirror must always be in another failgroup. So if you create proper failgroups, ASM can mirror according to your storage layout. Say your storage consists of four disk arrays (each with two disks) like on the picture below:

ASM failure groups

That is not yet the new thing, failgroups have been possible in 10g already. New is that you can now use the Fast Mirror Resync feature also on the failgroup layer with the 12c diskgroup attribute FAILGROUP_REPAIR_TIME. It defaults to 24 hours.

So if maintenance needs to be done with the disk array from the example, this can take up to 24 hours before the failgroup gets dropped.

I hope you found the explanation helpful, many more details are here :-)

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Oracle University Leadership Circle 2015

I’m in the Leadership Circle again :-) That is a quarterly Corporate Award for the best instructors worldwide according to customer feedback.

Oracle University has generally high quality standards, so it is hard to stand out individually. The more I am proud to be listed together with these great colleagues:

Oracle University Leadership Circle 2015

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uhesse.com in 2014 – a Review

Better than ever

2014 brought a new high water mark with 282,000 hits!

Meanwhile, uhesse.com gets over 1,000 hits per day on average.

Who are the referrers?

The vast majority of visitors came to uhesse.com via google searches: 163,000

On a distant second place is Twitter: 2,500 visitors came from there

Facebook and LinkedIn each lead 800 visitors to uhesse.com

400 visitors came from jonathanlewis.wordpress.com – thank you, Jonathan!

Where did uhesse.com refer to?

As a good employee, I sent 1,500 visitors to education.oracle.com and 1,300 to oracle.com :-)

About 600 each went to jonathanlewis.wordpress.com and blog.tanelpoder.com, while richardfoote.wordpress.com got 350 visitors

Nationality of the visitors

They came from almost everywhere, the full list contains 200 countries in total – isn’t the internet a great thing?

Vistors of uhesse.com by nationality

Thank you all for visiting – I hope you come back in 2015 again :-)

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