Thursday, May 18, 2006

VLDB with ASM?

Today I've been on the first day of the ASM course/workshop led by Martin Gosejacob. He is quite good and knows the stuff. Since I've been playing with it before and installed the practice clusters for it, I kind of knew general content. Nevertheless, I got enough new and interesting pieces of information. Today I will mention my concerns regarding managing large amount of storage with ASM. Later I will try to post some other interesting twists and bells that are not commonly mentioned.

Serious limitation of ASM - it doesn't have dirty region logs. Oracle claims that you don't need it as Oracle database is ASM aware and recovery on ASM level is not required. However, there is serious flaw in a way that resyncing/rebalancing/resilvering of a large part of diskgroup have to be done from scratch.

Imagine few TB database with data mirrored by ASM across two SAN boxes accessed via separate FC's and separate switches. You will need to organized all disks into 2 failure groups (each group with disks from the same SAN box) so that all extents are mirrored across two SAN boxes.

Maintenance or unplanned outage on one of those components will cause the whole mirror to be unavailable and ASM will try to rebalance but having no available disks in one of failure groups it won't get anywhere. Database operational status is not impacted at this point (unless you need to add new datafile or resize it - see below). Again, ASM will start "ejecting" failed disks and status of disks will be HUNG, i.e. stuck in attempt to drop from diskgroup. If multipathing is used and both switches are connected to both enclosures than switch or FC card failure is not that bad but still no protection from SAN outage or firmware upgrade.

Let's assume that access to the secondary box is back in 30 minutes and we are trying to put disks back. Since there is no change log, there is no way to perform fast resilvering. But this is not the full story. As mentioned before, ASM has already initiated removal of failed disks from diskgroup and the fact that disks are back doesn't affect ASM in any way now. You cannot drop failed disks until ASM finishes rebalancing, you cannot resync them as ASM is actually dropping them - no way back. The only way out is to add enough disks to the failure group with HUNG disks so that ASM can finish rebalancing and finally drop disks from failed SAN box.

This transforms in hours/days of rebalancing for several TB and additional space equal to the size of failed disks. In reality, it's probably possible to reuse failed capacity but it will include reorganizing it on SAN level, presenting as new devices, cleaning them up and adding as new disks. There are two options to add same disks:
1. Clean up ASM labels (dd zeroes to the beginning of the file about 1 MB) prior to adding.
2. Add disks to diskgroup with force option.
The first method requires additional efforts and the second is quite dangerous as one can force adding another good disk used somewhere else. Especially, if you use wildcards (would like to put all 100 devices manually?). Well, in the first case you may as well overwrite good disks.

As I mentioned already, if there is not enough free space for rebalancing in one of failure groups then additional space cannot be allocated - i.e. files cannot be extended and new files added in this diskgroup. So until enough space is provided, you are at risk of getting your tablespaces full.

Interesting what is going to happen with archive logs when one failure group is "out". Need to test it as I believe that new files will not be created until some disks are added.

In fact, it ASM with its current methodology couldn't really use dirty region logs principle. As soon as failure is detected, ASM initiates drop of those disks. To solve this issue, there should be an option to avoid automatic rebalancing in addition to introduction of dirty region logs.
So how to mitigate this problem with current ASM implementation?
It's possible to use many small storage boxes instead of couple high end enclosures. However, this will require more complex SAN networking with multiple switches and FC cards.


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