Re: very large data-offset?

[Duane <duane@evenson.tk>]

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On 2017-12-03 05:51 PM, NeilBrown wrote:
> On Thu, Nov 30 2017, Duane wrote:
>
>> Why is the data offset set so big? I created a 3x4TB RAID5 array and the
>> data offset was 128MB. Chunk size was the default 512kB.
> It is less than 0.1% of the device...
>
>> I cannot see why such a large offset is used. I would think the data
>> offset need only be at most the chunk size plus the space (1 sector) for
>> the superblock and bitmap.
> It is insurance.  If you want to change the chunksize later, having a
> lot of head-room will allow the reshape to go much faster.
>
>> When reshaping the array, I am prompted to use an external file, so I
>> don't see that mdadm ever uses the space.
> Citation needed.... what version of mdadm, what kernel?  What reshape
> command?
kernel:  9.64-1-lts
mdadm:  mdadm - v4.0 - 2017-01-09
action:  reduce the number of raid devices
>>
>> I tried making some test arrays and got much smaller sizes. A 3x1GB
>> RAID5 array with 64k chunks had a 1MB data offset.
>>
>>
>> If I make a 7x4TB RAID5 array with 64kB chunks, is there a problem with
>> setting the data offset to around 2MB?
> Only that it might reduce your options in the future, though probably
> not by much.
>
> NeilBrown


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