Need pointers to shrinking an array

["Michał Sawicz" <michal@sawicz.net>]

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Hi again,

Given an array with --examine like so:
> /dev/sdf1:
>           Magic : a92b4efc
>         Version : 1.1
>     Feature Map : 0x0
>      Array UUID : 9ce41f94:7489a7f2:6897e4b5:269c8b3b
>            Name : media:2  (local to host media)
>   Creation Time : Sun Jan 24 22:32:58 2010
>      Raid Level : raid6
>    Raid Devices : 10
> 
> Avail Dev Size : 1953519738 (931.51 GiB 1000.20 GB)
>      Array Size : 15628156928 (7452.09 GiB 8001.62 GB)
>   Used Dev Size : 1953519616 (931.51 GiB 1000.20 GB)
>     Data Offset : 264 sectors
>    Super Offset : 0 sectors
>           State : clean
>     Device UUID : 97945e4f:9f439aec:951cb289:47862ad7
> 
>     Update Time : Sun Sep 18 16:31:52 2011
>        Checksum : 4da159ea - correct
>          Events : 4381317
> 
>          Layout : left-symmetric
>      Chunk Size : 256K
> 
>    Device Role : Active device 7
>    Array State : AAAAAAAAAA ('A' == active, '.' == missing)

On it, a resized filesystem:
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/md2             1982282708 1970782580  11500128 100% /mnt/store.tmp

I now need to shrink the array to as few drives as possible. I know
`mdadm --grow --array-size=x` will let me check if the resulting FS is
still fine. Is x in Kibibytes, same as for --size? What's the best way
of determining x? 

Then, a `mdadm --grow --raid-devices=y` will let me reduce the number of
drives used. Is there a way to tell _which_ drives are to be active and
which - spares in the resulting array?

Is there anything else I should know?

Regards,
-- 
Michał (Saviq) Sawicz <michal@sawicz.net>

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