Re: Any way to Increase MD_SB_DISKS=27 ? I need 31 devices

[Tyler <pml@dtbb.net>]

Peter, after looking at the code after i sent the email, I more or less 
figured out what you put below, i'm wondering though, will changing the 
structures here affect anything else? other than the mdadm tool.. what 
about mkfs.ext3/xfs and possibly mount?

Thanks,
Tyler.

Peter T. Breuer wrote:

>Tyler <pml@dtbb.net> wrote:
>  
>
>>I'd like to build a Raid-6 array using 31 drives, is there a patch 
>>available or an easy way to safely increase the *MD_SB_DISKS*=27 to a 
>>higher number?  My attempts at patching the source myself were 
>>unsuccessful, more than likely because i don't know what/why the limit 
>>is 27, and whether that effects other parts of the kernel.
>>    
>>
>
>It's 27 because that's how much space there is in some of the structs.
>
>Look in include/linux/raid. md_p.h, for example.
>
>   #define MD_SB_DISKS                     27
>
>and it calculates how much space that needs:
>
>   #define MD_SB_DISKS_WORDS (MD_SB_DISKS*MD_SB_DESCRIPTOR_WORDS)
>   #define MD_SB_DESCRIPTOR_WORDS          32
>
>and all that is part of what must fit in a "mdp_superblock_s" struct.
>
>           /*
>            * Disks information
>            */
>           mdp_disk_t disks[MD_SB_DISKS];
>
>with the space at the tail of the struct calculated ...
>
>
>        /*
>         * Reserved
>         */
>        __u32 reserved[MD_SB_RESERVED_WORDS];
>
> ... by figuring how much space was used and subtracting from 1K:
>
>       #define MD_SB_RESERVED_WORDS            (1024 - MD_SB_GENERIC_WORDS - MD_SB_PERSONALITY_WORDS - MD_SB_DISKS_WORDS - MD_SB_DESCRIPTOR_WORDS)
>
>and the space used includes the MD_SB_DESCRIPTOR_WORDS conisting of
>those 27 32-byte array elements.
>
>To make this number bigger, you would first have to make more room in
>the struct, by changing the 1K in the calculation to 4096 (thus
>extending the struct by extending its reserved section), then you
>could use more of that room by increasing the 27. I presume the max
>would now be 3*32+27 = 123.
>
>
>There might be another such struct in the includes. I haven't looked
> ...
>
>Peter
>
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