stripe size checking (was "Re: about linear and about RAID10")

[David T-G <davidtg-robot@justpickone.org>]

Roger, et al --

...and then Roger Heflin said...
% How big is your stripe size set to?   The bigger the stripe size on the
% md40 main raid the closer it gets to linear.

I don't know ... and I don't know how to tell.

  davidtg@jpo:~> sudo mdadm -D /dev/md40
  /dev/md40:
	     Version : 1.2
       Creation Time : Mon Aug  8 12:15:12 2022
	  Raid Level : raid0
	  Array Size : 3906488320 (3.64 TiB 4.00 TB)
	Raid Devices : 2
       Total Devices : 2
	 Persistence : Superblock is persistent
  
	 Update Time : Mon Aug  8 12:15:12 2022
	       State : clean 
      Active Devices : 2
     Working Devices : 2
      Failed Devices : 0
       Spare Devices : 0
  
	      Layout : -unknown-
	  Chunk Size : 512K
  
  Consistency Policy : none
  
		Name : jpo:40  (local to host jpo)
		UUID : 4735f53c:7cdf7758:e212bec6:aa2942e8
	      Events : 0
  
      Number   Major   Minor   RaidDevice State
	 0       9       41        0      active sync   /dev/md/md41
	 1       9       42        1      active sync   /dev/md/md42
  
  davidtg@jpo:~> sudo mdadm -E /dev/md40
  /dev/md40:
     MBR Magic : aa55
  Partition[0] :   4294967295 sectors at            1 (type ee)

Is it the 512k chunk size?

% 
% And I think I did miss one behavior that explains it being faster with
% larger, and not sucking as bad as I expected.  On disks the internal cache
% is supposed to cache the entire track as it goes under the head, so when

Sounds familiar.


% you ask for the data theoriticaly requiring a seek the drive may already
% have the data still in its cache and hence not need to do a seek.   The
% performance would then be ok so long as the data is still in the cache.

Right.  Everything is awesome when the data is cached :-)


% But given your tests md40 at best gets close to the underlying raids
% performance.  When linear the underlying performance should be roughly
% equal.

Now that I understand linear, that makes sense.


Thanks again & HANN

:-D
-- 
David T-G
See http://justpickone.org/davidtg/email/
See http://justpickone.org/davidtg/tofu.txt