This discussion is archived
2 Replies Latest reply: Aug 10, 2012 1:17 AM by BillyVerreynne RSS

What  is /dev/mapper in layman's terms ?

C.Boutet Newbie
Currently Being Moderated
GI Version: 11.2.0.3 on RHEL 6 (still planning )

I have only worked in Solaris and AIX before. Our management has decided to do all new RAC implementations in OEL or RHEL.

Solaris is robust and easy to maintain !

Our storage team allocae LUNs based on WWN (sort of MAC address for our HBA cards in the Solaris server) and all shared LUNs are visible at /dev/rdsk location from both of our nodes (2-Node RAC)

I gather that in Linux, it is visible at /dev. I come across scary terms like +/dev/mapper+ , multipathing quite frequently with regards ASM for RAC.

What is /dev/mapper from a 'Solaris DBA' point of view ?

What is multipathing ?
  • 1. Re: What  is /dev/mapper in layman's terms ?
    Levi-Pereira Guru
    Currently Being Moderated
    C. Boutet wrote:
    GI Version: 11.2.0.3 on RHEL 6 (still planning )

    I have only worked in Solaris and AIX before. Our management has decided to do all new RAC implementations in OEL or RHEL.

    Solaris is robust and easy to maintain !

    Our storage team allocae LUNs based on WWN (sort of MAC address for our HBA cards in the Solaris server) and all shared LUNs are visible at /dev/rdsk location from both of our nodes (2-Node RAC)

    I gather that in Linux, it is visible at /dev. I come across scary terms like +/dev/mapper+ , multipathing quite frequently with regards ASM for RAC.

    What is /dev/mapper from a 'Solaris DBA' point of view ?
    There no another way to explain what is "Multipath". It's like try explaining what is "CPU" to an DBA, we use same way to explain what is "CPU" to everybody.
    All platform support multipath, you need find out how configure multipath on platform that you are using.

    ASM support multipathing in all platform. On linux we use the "/dev/mapper", on AIX the MPIO is transparent to ASM and so on.

    What is multipathing ?
    Multipathing is a software technology implemented at the operating system device driver level. Multipathing creates a pseudo device to facilitate the sharing and balancing of I/O operations across all of the available I/O paths. Multipathing also improves system performance by distributing the I/O load across all available paths. This provides a higher level of data availability through automatic failover and failback.

    Although ASM is not designed with multipathing functionality, ASM does operate with multipathing technologies. Multipathing technologies are available from many sources. Storage vendors offer multipathing products to support their specific storage products, while software vendors usually develop multipathing products to support several server platforms and storage products.


    Read it:
    http://en.wikipedia.org/wiki/Multipath_I/O

    Regards,
    Levi Pereira
  • 2. Re: What  is /dev/mapper in layman's terms ?
    BillyVerreynne Oracle ACE
    Currently Being Moderated
    C. Boutet wrote:

    I have only worked in Solaris and AIX before. Our management has decided to do all new RAC implementations in OEL or RHEL.
    Solaris is robust and easy to maintain !
    So is Linux. So what is your point exactly?
    Our storage team allocae LUNs based on WWN (sort of MAC address for our HBA cards in the Solaris server) and all shared LUNs are visible at /dev/rdsk location from both of our nodes (2-Node RAC)
    Exactly the same concept on Linux. Only the LUNs are visible (using Multipath) as devices in +/dev/mpath+.
    I gather that in Linux, it is visible at /dev.
    Correct. LUNs are visible as scsi devices to the kernel. If there are multiple I/O paths to that LUN (typically the case using dual port HBAs and HCAs), that LUN will be seen more than once.

    For each unique I/O path to that LUN, there will be a scsi +/dev+ entry. Due to the I/O fabric layer's enumeration of LUNs, these scsi +/dev+ entries will also not be named the same after each reboot. For example, LUN1 can be seen as devices +/dev/sdg+ and +/dev/sdh+ and then, following a reboot, as +/dev/sdj+ and +/dev/sdk+. You cannot therefore rely on the +/dev+ entries for consistent and persistent device names - and this is perfectly okay, as that is simply how the I/O fabric layer works - even for Solaris.

    However, the WWN for each of these devices will however be the same. So if +/dev/sdj+ and +/dev/sdk+ have the same WWN, then it is the same LUN.

    A s/w layer is therefore required to deal with the multiple I/O paths to the same LUN via different (non persistent) +/dev+ devices. EMC provides the s/w called PowerPath that does this.

    From Linux clusters (and Linux clusters dominates over 90% of the world's 500 fastest supercomputer clusters), an Open Source project called Multipath arose that also does this. Multipath is robust and mature and ships as a default multiple I/O path driver for Linux Enterprise distributions.

    Multipath does 2 simple things:
    a) it enables you to give a fixed/persistent multipath device name to a WWN
    b) it maps the physical +/dev/+ devices to that multipath device

    Of course, Multipath does more than this. You can blacklist WWNs that should never be used. You can specify different path policies, default device selector algorithms and so on.

    Multipath is used in real-world 2000+ node Linux clusters addressing several PB of SAN storage. So it is very robust.

    Oracle RAC and ASM runs on top of the multipath devices - be that Linux Multipath, EMC Powerpath, or whatever multipath I/O s/w layer used.

Legend

  • Correct Answers - 10 points
  • Helpful Answers - 5 points