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How do I compute data loss risk (MTTDL) for HDP pools that use DDP raid groups

  • 1.  How do I compute data loss risk (MTTDL) for HDP pools that use DDP raid groups

    Posted 08-06-2025 09:22
    Edited by System 10-17-2025 15:02
    When a storage administrator needs to compare the data loss risk of various storage system configurations
    the only available / standard metric is MTTDL.
     
    There are online tools for computing / comparing  MTTDL for storage 
    system configurations that use conventional RAID groups. For example, the "wintelguy" site
    has the raidmttdl.pl tool.
     
    I need a tool that will allow me to compute MTTDL for a DDP backed HDP pool
    based on selected raid_type, number of DDP-groups in the HDP pool ... etc
     
     
    Example:
     
    A.  1 HDP pool with the following 1 DDP group
         24 drives, raid_type 14D2P
     
     
    B.  1 HDP pool with the following 1 DDP group
         24 drives, raid_type 6D2P
     
    C.  1 HDP pool with the following 2 DDP groups
         12 drives, raid_type 6D2P
         12 drives, raid_type 6D2P
     
     
    It would also be good to understand how the risks of DDP
    compare to those of conventional RAID.
    Conventional RAID allows admins to upscale spare capacity
    (by defining multiple SPARE-DRIVES that can protect any RG in the system) 
    DDP does not appear to allow admins to define multiple SETS-OF-SPARE-CHUNKS"
    So, it appears (qualitatively), on the surface, that DDP (and similar schemes from other vendors)
    are somewhat less     robust that conventional raid systems, in multi-drive failure scenarios.


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    Andrew Romero
    HDSFC1159
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