to partition or not to partition (was "Re: striping 2x500G to mirror 1x1T")

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

Hi again, all --

...and then davidtg-robot@justpickone.org said...
% 
% I have a 1T data drive (currently in use with data) that I'd like to
% mirror with a pair of 500G drives striped together.  I'll be mirroring
% two partitions, and I'll be striping partitions to ensure the correct
[snip]

Is it OK to mirror a partition and an entire device?  Are there any pros
or cons?

I now have my 2ea 500G drives partitioned

  jpo:~ # parted /dev/sdc unit MiB print
  Model: ATA WDC WD5000AAVS-0 (scsi)
  Disk /dev/sdc: 476940MiB
  Sector size (logical/physical): 512B/512B
  Partition Table: gpt
  Disk Flags: 

  Number  Start      End        Size       File system  Name                    Flags
   1      1.00MiB    472171MiB  472170MiB               raid0-WD-WMASU0686160   raid
   4      472171MiB  476940MiB  4769MiB    reiserfs     wwn-0x50014ee055b724ad

  jpo:~ # parted /dev/sdd unit MiB print
  Model: ATA WDC WD5000AAVS-0 (scsi)
  Disk /dev/sdd: 476940MiB
  Sector size (logical/physical): 512B/512B
  Partition Table: gpt
  Disk Flags: 

  Number  Start      End        Size       File system  Name                    Flags
   1      1.00MiB    472171MiB  472170MiB               raid0-WD-WMASU0769665   raid
   4      472171MiB  476940MiB  4769MiB    ext3         wwn-0x50014ee055b72bcd

(I always reserve a small slice at the end of the disk in which to keep
array structure data and other helpful info) and striped 

  jpo:~ # mdadm --detail /dev/md0
  /dev/md0:
             Version : 1.2
       Creation Time : Wed Mar  9 05:48:41 2022
          Raid Level : raid0
          Array Size : 966739968 (921.96 GiB 989.94 GB)
        Raid Devices : 2
       Total Devices : 2
         Persistence : Superblock is persistent

         Update Time : Wed Mar  9 05:48:41 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:500Graid0md  (local to host jpo)
                UUID : fc674dbd:577b7d48:5cbe936f:6954eb62
              Events : 0

      Number   Major   Minor   RaidDevice State
         0       8       33        0      active sync   /dev/sdc1
         1       8       49        1      active sync   /dev/sdd1

into a 1T-ish device ready to be half a mirror.  I had originally done
lots of partition size math equal exactly half of my 1T drive partition

  jpo:~ # parted /dev/sdb unit MiB print
  Model: ATA ST31000524AS (scsi)
  Disk /dev/sdb: 953870MiB
  Sector size (logical/physical): 512B/512B
  Partition Table: gpt
  Disk Flags: 

  Number  Start      End        Size       File system  Name  Flags
   1      1.00MiB    858307MiB  858306MiB  xfs          pri
   4      950055MiB  953869MiB  3814MiB                 pri

with existing data (and having to fudge around, not surprisingly, the
striping metadata overhead), and so I've been playing

  jpo:~ # parted /dev/md0 unit MiB print
  Model: Linux Software RAID Array (md)
  Disk /dev/md0: 944082MiB
  Sector size (logical/physical): 512B/512B
  Partition Table: gpt
  Disk Flags: 

  Number  Start      End        Size       File system  Name         Flags
   1      1.00MiB    934641MiB  934640MiB               500Graid0md  raid
   4      934641MiB  944081MiB  9440MiB                 pri          msftdata

with how to slice up the stripe device to size it to match the existing
sdb1 partition with the mirroring metadata overhead, when I wondered ...
do I care?  I don't need the s4 slice on this virtual device; that's just
leftover space.  I hate to think of it going to waste, but it's really
trivial in the grand scheme of things.  Soooooo ... if it's just as happy
to mirror against a whole device as it is a slice of one, then maybe I
can skip ahead.

Thoughts?


Thanks again! :-)

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