A complete review, recommendation usage
About Oracle ASM Filter Driver
Oracle ASM Filter Driver (Oracle ASMFD) is a kernel module that resides in the I/O path of the Oracle ASM disks. Oracle ASM uses the filter driver to validate write I/O requests to Oracle ASM disks.
Oracle ASMFD simplifies the configuration and management of disk devices by eliminating the need to rebind disk devices used with Oracle ASM each time the system is restarted.
Oracle ASM Filter Driver rejects any I/O requests that are invalid. This action eliminates accidental overwrites of Oracle ASM disks that would cause corruption in the disks and files within the disk group. For example, the Oracle ASM Filter Driver filters out all non-Oracle I/Os which could cause accidental overwrites.
Note : AFD is not compatible with ASMLIB and these packages can't exist together on a OS.
1) Starting with Oracle Grid Infrastructure 12C Release 1 (12.1.0.2), Oracle ASM Filter Driver (Oracle ASMFD) is installed with an Oracle Grid Infrastructure installation.
2) Oracle Automatic Storage Management Filter Driver (Oracle ASMFD) rejects write I/O requests that are not issued by Oracle software. This filter helps to prevent users with administrative privileges from inadvertently overwriting Oracle ASM disks, thus preventing corruption in Oracle ASM disks and files within the disk group. For disk partitions, the area protected is the area on the disk managed by Oracle ASMFD, assuming the partition table is left untouched by the user.
3) Oracle ASMFD simplifies the configuration and management of disk devices by eliminating the need to rebind disk devices used with Oracle ASM each time the system is restarted.
4) ASMFD is a superset of ASMLIB; therefore it includes base-ASMLIB features (permissions persistence & sharing open handles).
5) The Oracle ASM filter driver (ASMFD) is a kernel module that resides in the I/O path of the Oracle ASM disks. Logically, ASMFD provides an interface between Oracle binaries and the underlying operating environment which includes the storage hardware interfaces. Following are descriptions of the key capabilities of ASMFD:
• Reject non-Oracle I/Os.
As a manager of Oracle storage, ASM is exposed to the capabilities of the Operating System (OS) when it comes to dealing with storage devices. In particular, non-Oracle commands have the ability to overwrite the contents of ASM disks which may lead to unrecoverable data loss. ASMFD only allows writes using an Oracle-specific interface and prevents non-Oracle applications from writing to ASM disks. This protects ASM from accidental corruption.
• Reduce OS resource usage.
An ASM instance contains numerous processes, or threads on Windows. Without ASMFD, each process that is I/O capable needs to have its own dedicated open file descriptor for each disk. When a database has thousands of processes accessing hundreds of disks, there is an explosion of file descriptors leading to considerable OS resource consumption. ASMFD exposes a portal device that can be used for all I/O on a particular host. The same portal device can be shared by all the processes associated with multiple database instances. The result is that using ASMFD, the required number of open file descriptors is drastically reduced.
• Enable device name persistence.
ASMFD do not require additional configuration to ensure device name persistence by using udev rules or third-party storage drivers.
• Faster node recovery.
With the current implementation of Oracle Clusterware, init.d scripts panic the node in situations when Cluster Synchronization Services (CSS) is not functioning correctly. Using this mechanism, nodes are fenced to ensure the integrity of the rest of the cluster. While effective, this solution is costly because of the time required to reboot the node and restart all the required processes. ASMFD allows Oracle Clusterware to perform node level fencing without a reboot. So with ASMFD, it is possible to achieve the same result by restarting the Oracle software stack instead of rebooting the entire node. This process is just as effective, but far quicker.
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ASMLIB:
What is ASMLib?
ASMLib is an optional set of tools and a kernel driver that can be inserted between ASM and the hardware, as well as an application library used by the Oracle database software to access ASM disks. It is a support library for the ASM feature of Oracle 10g and higher ,single instance database servers as well as RAC installations. ASM and regular database instances can use ASMLib as an alternative interface for disk access. ASMLib has three components:
Kernel driver – oracleasm is a Linux kernel driver also known as the Oracle ASMLib kernel driver. This is an open-source (GPL) kernel driver and is available from Oracle as source and binary RPMs. Note that although this driver is provided under an open source license, it has not been accepted into the mainline Linux kernel.
Support tools – oracleasm-support provides the utilities to manage the ASM library driver. Oracleasm-support is an open-source package (GPL) and is available from Oracle as source and binary RPMs.
Application library – oracleasmlib package provides the actual ASM library. This is a closed source, binary-only RPM, available as a free download from Oracle. Oracle introduced ASMLib in 2004 to provide Oracle workloads on Linux with performance and stability comparable to equivalent workloads on UNIX. ASMLib addressed deficiencies – including a lack of async and direct I/O -- that existed in the 2.4 and higher Linux kernel. The Linux community addressed these deficiencies in the 2.6 Linux kernel with the addition of udev and device-mapper multipath which together with LVM provided native multipathing, scalable native volume management, and persistent device naming. Additionally, the 2.6 Linux kernel added tools for handling large numbers of disks, especially SAN-attached disks. How is ASMLib delivered? ASMLib is available as a free download from the Oracle ULN website. Unlike ASM, ASMLib is not included with the Oracle database product.
ASMLib is available for Linux only; it does not exist for any other platform.
When would I want to use ASM and ASMLib together?
What are the advantages and disadvantages?
Oracle recommends using ASM with ASMLib or AFD together for better manageability and persistent device naming. Note that Oracle makes no claims that ASM with ASMLib delivers performance benefits over ASM without ASMLib.
Advantages
• Perceived better manageability.
• Well documented and recommended by Oracle.
• Some Oracle DBAs and SysAdmins are trained in how to use ASM with ASMLib and are comfortable with this environment.
• Optimized for database applications via direct and async I/O provided by the ASMLib kernel driver.
Note Red Hat Enterprise Linux kernel 2.6.X and higher supports optimized direct and async I/O for all supported filesystem EXT3/4, XFS, GFS and NFS.
Disadvantages
• Requires ASMLib kernel driver that is not included in the mainline Linux kernel.
• ASMLib delivers no known performance benefits.
• Red Hat Enterprise Linux with ASMLib does not have government security certification.
• ASMLib is not compatible with SELinux.
• ASMLib is not multipath aware and can silently choose to work through a single path, causing performance and reliability issues.
• Non POSIX system calls for device access lead to issues with trouble-shooting and performance monitoring.
Can I run an Oracle single instance database with ASM and without ASMLib?
What are the advantages and disadvantages? What are the alternatives?
Yes. It is possible to run an Oracle Single Instance database with ASM and without ASMLib by specifying udev rules and/or using device mapper multipathing to achieve persistent device naming, both of which are a standard part of Red Hat Enterprise Linux 5 and Red Hat Enterprise Linux 6 and higher .
ASM can use the following for storage resources:
• Block devices (SATA, SAS, FC, iSCSI, FCoE, regular LUNs and LUNs with thin provisioning) with udev naming and/or multipathing.
• Raw Devices. • NFS v3 with verified NAS vendors.
Advantages
• Using ASM without ASMLib avoids the overhead resulting from an extra, non-mainline Linux kernel driver.
• Supported and documented by Oracle.
• Using ASM without ASMLib provides the ability to use operating system's native I/O multipathing.
• Using POSIX system calls for disk access enables improved performance monitoring and troubleshooting.
Disadvantages •
Customers may have standardized on Oracle deployments using ASMLib.
Can I use the Oracle single instance database without ASM or ASMLib ? What are my alternatives? How do I set it up?
Yes. There are two alternatives to using the Oracle single instance database without ASM or ASMLib:
• Use a local file system such as ext4.
• Use logical volumes such as provided by lvm2 with dm-multipath or an optional third party commercial multipathing software. Red Hat will address this configuration in a forthcoming reference architecture whitepaper.
Can I use Oracle Real Application Clusters (RAC) with ASM and without ASMLib?
What are my alternatives? How do I set it up?
ASMLib is an optional component of either an Oracle single instance or RAC configuration. Below are alternatives to using ASMLib for RAC:
• ASM with LVM: ASM + block devices + udev naming + multipathing.
• Cluster file systems with cluster volume manager: gfs/clvm (Red Hat Enterprise Linux 5) or Symantec Cluster File System and Symantec Cluster Server (http://www.symantec.com/business/storage-foundation-for-oracle-rac).
• NFS v3 with verified NAS vendors.
Red Hat will address this configuration in a forthcoming reference architecture whitepaper.
Can I use Oracle Real Application Clusters (RAC) without ASM or ASMLib? What are my alternatives? How do I set it up?
Yes a preferred option is Udev rules.
Although most Oracle RAC installations with SAN-attached storage use ASM, RAC can be deployed without ASM if database files are located on an NFS server, certified cluster file system, or raw devices. (Note that Oracle discourages the use of raw devices.) Red Hat will address this configuration in a forthcoming reference architecture whitepaper.
I already use ASM and ASMLib. How do I migrate from using them to something else?
This is a simple migration.
Perform the following:
1. The DBA stops all database instances using storage through ASM and ASMLib.
2. The system administrator configures udev (or multipath) to assign permissions allowing ASM processes to have read and write access to ASM devices
3. The DBA changes one ASM parameter to point ASM to the assigned devices.
4. The DBA restarts the ASM instance and databases.
5. As with all data-related changes, a backup is highly recommended.
Some theoretical benefits of ASMLIB API:
- always uses direct, async i/o
- solves persistent device naming, even if underlying device moves across reboots
- solves file permissions and ownership
- reduced user mode to kernel mode context switches during I/O, possibly reducing CPU usage
- reduced file handle usage
- pass metadata such as I/O prioritization to storage device (don’t think this is implemented in the Linux version)
Note :
using direct i/o and async i/o have dependencies on Oracle parameters Disk_asynch_io =true and filesystemio_options=Set All .
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UDEV Rules:
What is Udev?
Udev is the mechanism used to create and name /dev device nodes corresponding to the devices that are present in the system. Udev uses matching information provided by sysfs with rules provided by the user to dynamically add the required device nodes.
Udev rules doesn't Linux kernel limitations on upgrade time instead of ASMLIB and AFD kernel dependencies.
All udev file should be under /etc/udev/rules.d/
basic format for udev rule is.
key1=”value”, key2=”value”, … keyN=”value”, name=”value”, symlink=”value”
You can find the key value by using udevadm command. Lets understand the environment key.
# multipath -ll mpathxy
mpathxy (wwn) dm-6 (vendor)
size=600G features=’1 queue_if_no_path’ hwhandler=’0′ wp=rw
`-+- policy=’service-time 0′ prio=1 status=active
|- 1:0:0:6 sdj 8:1 active ready running
|- 1:0:1:6 sdk 65:2 active ready running
|- 2:0:0:6 sdl 65:14 active ready running
`- 2:0:1:6 sdm 66:24 active ready running
So we have multipath mpathxy and reference device mapper is dm-6.
# udevadm info –query=all –path=/devices/virtual/block/dm-6
Above command will give you list of variables which you can use in udev rules.
You can make the following entry in /etc/udev/rules.d/99-oracle-asmdevices.rules
KERNEL==”dm-*”,ENV{DM_UUID}==”mpath-wwn”,OWNER=”username”,GROUP=”groupname”,MODE=”0660″
Or you can use following script to create the /etc/udev/rules.d/99-oracle-asmdevices.rules
Generate udev rules for Physical and virtual server
/etc/udev/rules.d/99-oracle-asmdevices.rules
KERNEL=="dm-*",ENV{DM_UUID}=="mpath-wwn",OWNER="oracle",GROUP="oinstall",MODE="0660"
KERNEL=="dm-*",ENV{DM_UUID}=="mpath-wwn1",OWNER="oracle",GROUP="oinstall",MODE="0660"
# /sbin/udevadm control –reload-rules
# /sbin/udevadm trigger –type=devices –action=change
Now verify the udev rules via
# ls -l /dev/dm*
UDEV Rules Advantages:
• Using ASM without ASMLib avoids the overhead resulting from an extra, non-mainline Linux kernel driver.
• Supported and documented by Oracle.
• Using ASM without ASMLib provides the ability to use operating system’s native I/O multipathing.
• Using POSIX system calls for disk access enables improved performance monitoring and troubleshooting.
. There are no issues linux kernel upgrades on Asmlin & Asm filter drive.
UDEV Rules Disadvantages:
• Customers may have standardized on Oracle deployments using ASMLib.
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♨️My experience:
You may find i/o tests and some benchmarks on using ASMLIB or ASM Filter Drive that speaks about performance better as a result .
Normally these i/o testing leads to AFD has better performance instead of ASMLIB. And ASMLIB usage is better performance instead of UDEV Rules.
Generally, ASM Filter Drive is very dependent on Grid version and not always available to you, but its features is noticeable and seductive for DBA.
Although isolation levels , security features on AFD is a good feature, But you should consider that these performance benchmarks is very dependent on hardware and resources, so this not meaning that using them also is better performance for your environment, finally with attention to linux kernel and oracle grid upgrades and issues on AFD and ASMLIB, you can consider to use only UDEVs + Multipathing on Linux and DBA must consider maintenance cost of there tools with respect to its features also.
Therefore , on new kernel and Grid versions that support AFD, you can use it and enjoy its features and when Grid version not supported you can use ASMLIB, but it is possible you force to change and using Udev rules in the specific time in future cause of upgrade your linux or database.
So dont worry about performance effective on using Udev rules instead of ASMLIB or AFD, if realy there tools give us a performance rate, this feature instead of minimizing maintenance cost on future is a right and rational decision.
About Migrating to Oracle ASM Filter Driver From ASMLIB
If Oracle ASMLIB was installed, but not used earlier, you must create disk labels to enable migration of Oracle ASM disk groups to Oracle ASM Filter Driver (Oracle ASMFD) after installing Oracle Grid Infrastructure 12c Release 1 (12.1.0.2).
Oracle recommends that you temporarily move Oracle Cluster Registry (OCR) and voting files to another disk group if one is available, as described in "https://docs.oracle.com/en/database/oracle/oracle-database/12.2/ostmg/administer-filter-driver.html#GUID-9C370239-C67D-4813-BE83-B0528397C02D
and migrate the disk group to use Oracle ASMFD. After migrating the disk group to use Oracle ASMFD, move OCR and voting files back to the disk group. You can similarly migrate any other disk groups if they contain OCR or voting files to ensure online migration of all disk groups to Oracle ASMFD.
Regarding
Alireza Kamrani.
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