[Lustre-discuss] clarification on mkfs.lustre options

[Sebastian Gutierrez]

Hello 

This will probably be a pretty basic question however I wanted to verify that I was understanding the documentation correctly. 

The enclosure I am using supports 15 disks.   We have initially ordered 6 disks + hostpare.  I think my recommendation is going to be a 6 disk raid 6.  Which will give us a 4 data disk raid set plus 2 parity disks.  Later we will have the option to create another 6 disk raid6 or expand the current raid set to a 10 disk raid 6 then move the journal to a 4 disk raid 1/0 and have 1 disk hot spare.   

The current Raid 6 will have a 128k chunksize (lustre terminology) 

this gives us:

 <stripe_width> = <chunksize> * (<disks> - <parity_disks>) <=1MB
512K <= 128k*4

<chunksize> <= 1024kB/4; either 256k, 128k, 64k 
256k = <	= 1024k/4k

<chunk_blocks> = <chunksize(decided above)> / 4k
32 = 256/4 

128k = 512k / 4k
<stripe_width_blocks> = <stripe_width> / 4k   

Therefore 
My mkfs options should be 
--mkfsoptions="-E stripe=128 -E stride=32" /dev/sdb

upgrade options:
The filesystem chunk_block options would still be valid since the chunk_blocks would stay the same. 

purchase 6 more disks
This will allow for a new raid 6 that is aligned the same way with 3 hotspares. 

or (I need clarification if this understanding is correct)

purchase 10 more disks
Expand the current raid 6 to a larger 10 disk/raid 6 with a 4 disk raid 1/0 for a external journal plus a hot spare.  

Is my understanding of the documentation accurate? 
Do both of these options seem like potential upgrade options?

Cheers, 
Sebastian