Oracle ASM in Azure corruption - follow up

A few weeks ago i was contacted by the ASM product manager in a follow up to my post on the corruption issues i'd had with ASM in Azure. We had a conversation about the issues that I'd had and he told me that Oracle were going to be doing some more investigation into the issue with Microsoft as  it was believed that the issue really shouldn't have happened in the first place (even ignoring my workaround). A couple of weeks ago he gave me an update and I'm sharing that email below (Jim - i think it sounds better to include most of the email as it has some extra explanation info that people may fine interesting - i took out your email address to avoid you getting spammed by everyone :-))


The last time we spoke, I indicated that Oracle would be doing independent testing of the corruption issue you first reported in your blog. Today, I would like to bring you up to date about what we found and also request an update to your blog to reflect our understanding of the problem.

The testing was a joint effort between Microsoft and Oracle. Microsoft's engineering team provided  the identification of the critical bug. The testing began by reproducing the corruption with much of the same configuration reported by you. We were readily able to reproduce the exact same corrupted data pattern. Our testing also revealed that certain Linux kernels experienced this problem while others did not have a problem. Microsoft seemed to have a pretty good idea of what the problem was and provided Oracle a patch. 

With the Microsoft supplied Linux kernel patch we could not recreate the corruption regardless of load placed on the database. The patch modifies memory block management handling in the kernel running in the VM associated with doing IO. The problem scenario is that when an Oracle database utilizing ASM runs in an Azure VM, Microsoft's paravirtualization driver (storvsc), running in the VM, interacts with the kernel IO buffer logic to cause wrong data to be written by the database log-writer. The nature of the corruption is unpredictable and happens infrequently, however under heavy load, without the patch the corruption is easily reproduced.

The nature of the bug is that errors are not reported at the moment of the corruption. It is only later when database's archive process reads the redo logs that the corruption is detected and reported. It is not known if there are other silent data corruptions occurring to other files and are simply not reported.

There are a number of circumventions reported by you and others, including not using ASM for database storage, treating the ASM disks as 4K sector devices, avoiding the 3.10.0-514 Redhat kernel, and using ASMLIB for device management. At this point, if at all possible, Oracle recommends customers simply avoid this particular kernel in an Azure environment. They could use Oracle's UEK kernel (Oracle Linux Azure VM) or an older Redhat kernel. The other workarounds of treating ASM disks as 4K sector disks and/or using ASMLIB, while likely effective, do involve additional management efforts, and we're not completely certain that the issue is entirely avoided. 

With respect to your blog, I request that it be updated to include the following points:

·         LUNs in an Azure are presented as 512e devices. That means that internally they are structured as 4K "sector" disks (physical), but emulate 512-byte sector disks (logical) from an application perspective. I put "sector" in quotes because this is not really true of SSD disks, but the Advanced Format Disk (512e) specification was written with conventional rotating disks in mind. ASM and the Oracle database work correctly with Advanced Format disks in 512-byte emulation mode, and it is not necessary to create 4K disk groups for correct operation. Some flash storage vendors recommend doing so, but strictly for performance reasons associated with their particular products. There is no need for ASM to detect sector size in this respect.
·         The bug discussed here is with particular Redhat kernels. This bug is exposed in an Azure virtualization environment with ASM. The kernel we know to be problematic is 3.10.0-514. There may be other kernels having the problem, but we could not reproduce the issue with Redhat kernel 3.10.0-327 or Oracle’s current UEK kernel.
·         We do not know if creating 4K sector disk groups is a complete fix. At best it circumvents the bug. Our testing seems to verify it as a reliable workaround, but there may be situations where data is still silently corrupted.

Thank you for reporting this issue. Oracle takes the issue of data corruption as one of our most important concerns. If there is other information I can provide or if you would like to discuss this by phone, please let me know.



  2. Thanks for link up to the redhat note on this

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