On 2017-11-30 03:45 AM, Wols Lists wrote:
On 30/11/17 10:26, 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.
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.
When reshaping the array, I am prompted to use an external file, so I
don't see that mdadm ever uses the space.
Do you mean the manual tells you, or that mdadm refuses to run otherwise?
Certainly with a new array on larger disks, a backup file should be
totally unnecessary.
Run a reshape of some sort, and see if the offset changes :-)
Reshaping won't run and suggests the use of the backup file.
I like the way mdadm gives helpful hints rather than fails quietly. :)
I don't have the space right now to grow the array device sizes and
watch for changes in the data offset.
It is going on my todo list for when I've quit reshaping my filesystem
and things have stabilized.
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?
Dunno. How do you know the bitmap is "just one block"? I haven't dug in
to it so I don't know, but it makes sense to me that as the disks get
bigger, so does the bitmap.
I misspoke. I wasn't paying too much attention to bitmaps: just saw a
value for bitmap offset in the header and assumed I had a bitmap.
Further investigation due to your question showed that I don't have a
bitmap by default.
What's the point of fighting the defaults, anyway? Just add
another/bigger disk if you need more space.
I'm poor; 5, now 6 disks are my budget's limit. :(
Cheers,
Wol
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