Re: btrfs on software RAID0

[Marc MERLIN <marc@merlins.org>]

On Tue, May 06, 2014 at 09:02:46AM +0200, john terragon wrote:
> just one last doubt:
> 
> why do you use --align-payload=1024? (or 8912)
> Cryptsetup man says that the default for the payload alignment is 2048
> (512-byte sectors). So, it's already aligned by default to 4K-byte
> physical sectors (if that was your concern). Am I missing something?

With 4K sectors, I agree that 2048 would be better.

What I was trying to do there is avoid write amplification.

After reading 
http://wiki.drewhess.com/wiki/Creating_an_encrypted_filesystem_on_a_partition
I went with
mdadm --create /dev/md8 --level=5 --raid-devices=5 /dev/sd[abdef]1 --chunk=256 --bitmap=/boot/bitmap-md8
which I believe required me to use
cryptsetup luksFormat --align-payload=1024 -s 256 -c aes-xts-plain64  /dev/md8
(that was with 5 drives, or 4 drives with data).

Would agree with the math?

If so, for 4K sector sizes, if we have to use align-payload=1024, in
turn I'd have to use --chunk=512. 
Does that sound right?

Thanks,
Marc
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