当前位置: 首页 > EMC > 正文

Using EMC Powerpath with ASMLib

ASMLib is optional software to simplify the use of Automatic Storage Management (ASM) for Oracle databases on Linux.

ASMLib ensures consistent naming of devices across RAC clusters, and also maintains permissions on devices across reboots, a feature that was important until UDEV rules were added to Linux with the 2.5 kernel.

EMC Powerpath is an advanced multi-pathing host based solution that works with EMC arrays to intelligently load balance I/O across all available paths as well as provide fault tolerance by automatically re-routing traffic around failed paths. EMC Powerpath is significantly more powerful and robust that native Linux MPIO.

One reason why many DBAs moved away from ASMLib was that with the RHEL 6.0 release, Oracle withdrew access to ASMLib to customers who did not purchase Oracle Linux support. Oracle has now reversed course on that decision, and with the RHEL 6.4 release ASMLib is again free to download and use for Oracle customers (see My Oracle Support Doc ID 1089399.1)

ASMLib has many advantages but also adds some complications, and the DBA needs to be aware of these, especially when using a multi-pathing solution like EMC Powerpath.

A feature of ASMLib is that devices stamped for ASM are assigned an alias. Each disk will be added to the /dev/oracleasm/disks/ device directory, and presented to ASM using the alias ORCL:diskname.

When Linux sees multiple paths to the same disk or LUN, it creates an entry in the SCSI device table for each path. Therefore a single LUN with two paths may appear to Linux as both /dev/sdg and /dev/sdh.

This is problematic for ASM, since ASM cannot natively handle two or more devices mapping to the same LUN.

Like ASMLib, EMC Powerpath also creates aliases, called pseudo-devices. One pseudo-device is assigned to cover all SCSI devices or paths that map back to the same LUN. EMC Powerpath pseudo-devices will have the naming convention /dev/emcpowerX where X is the device letter.

To see how pseudo-devices map to SCSI devices, use the following command:

 

This will produce output similar to the following:

 

In the above example, the pseudo-device /dev/emcpowera maps to the Linux SCSI devices sdas, sdae, sdq and sdac.

To gain the benefits of EMC Powerpath with ASMLib therefore, we need to stamp the pseudo-device as an ASM disk, not the individual Linux SCSI devices. The ASMLib software must be configured to scan the pseudo-devices and not the SCSI devices, which is achieved by editing the file /etc/sysconfig/oracleasm.

The directives ORACLEASM_SCANORDER and ORACLEASM_SCANEXCLUDE need to be modified as follows:

 

Now when ASMLib starts, it will scan for ASM disks among the Powerpath pseudo-devices instead of the SCSI devices.

Since ASMLib requires a partition table, we can create one on any of the devices covered by the pseudo-device, it will be visible on all devices as well as the pseudo-device.

 

We can now stamp the pseudo-devivce as an ASM disk using ASMLib:

 

To ensure that ASM is using the EMC Powerpath pseudo-devices and not the Linux SCSI devices we can verify the mapping using the following Korn shell script, which must be executed as root:

 

Executing this on a properly configured system should produce an output similar to this:

本文固定链接: http://t.yjsec.com/index.php/2017/12/06/54/ | 下一站

该日志由 admin 于2017年12月06日发表在 EMC 分类下, 你可以发表评论,并在保留原文地址及作者的情况下引用到你的网站或博客。
原创文章转载请注明: Using EMC Powerpath with ASMLib | 下一站
【上一篇】
【下一篇】

Using EMC Powerpath with ASMLib:等您坐沙发呢!

发表评论

快捷键:Ctrl+Enter