Re: Mdadm, udev and fakeraid?

On Sat, Apr 23, 2011 at 4:37 AM, Dan Williams [off-list ref] wrote:

On Sun, Apr 17, 2011 at 5:38 PM, NeilBrown [off-list ref] wrote:

quoted

On Fri, 15 Apr 2011 16:15:50 +0200 Seblu [off-list ref] wrote:

quoted

On Tue, Apr 5, 2011 at 8:20 AM, NeilBrown [off-list ref] wrote:

quoted

On Sun, 3 Apr 2011 18:03:50 +0200 Seblu [off-list ref] wrote:

quoted

Hello,

In the following commit, udev rules load isw_raid (fakeraid). From my
test, this doesnt work. I have to call dmraid to have something
working.
http://neil.brown.name/git?p=mdadm;a=commit;h=475a01b8bce8575dd1b2ab6495e65e854702ac0e

isw_raid is only fakeraid devices? mdadm is able to mount fakeraid partition?

I'm sorry but I cannot parse those questions successfully so I'm not sure
what you are asking.

Hello Neil,

in my previous mail, i used word fakeraid about raid created with
dmraid and i used softraid about raid created with mdadm. it was not
clear.

So my question was about compatibily. Raids created by dmraid can be
assembled with mdadm and vice versa?

quoted

Both dmraid and mdadm can manage some 'fakeraid' arrays.  dmraid supports a
wider variety.  mdadm supports raid1 and raid5 more completely than dmraid
does.

mdadm -> create soft raid for linux  (now there is new format: ddf and imsm) ?
dmraid -> create soft raid from industry raid card format  ?

No, it isn't that simple.

dmraid uses the 'dm' kernel module.  mdadm uses the 'md' kernel module.

As such dmraid doesn't support RAID5 (yet) and doesn't support RAID1 very
well.
mdadm supports both of these well, but doesn't support the same range of
"industry raid card formats".

There is a growing amount of overlap.

quoted

quoted

Both should support isw to some degree.
Intel are currently working with mdadm to make it provide full support for
"IMSM" (Intel Matrix Storage Manager).  I don't know the exact relationship
between 'isw' and 'IMSM' - maybe they are different names for the same thing.

ok

quoted

If mdadm doesn't work for your isw arrays, and you want it to, then I suggest
you report details about what is, or is not, happening.

My purpose is to improve archlinux startup detection of fakeraids
(mdadm + dmraid).

With mdadm everything works correctly without call to "mdadm -As"
With dmraid, no raid is created by udev rules, so we need to run
"dmraid -i -ay" at startup.

To test this kind of raid, i created a dmraid array in a vm. This
created me a /dev/mapper/isw_bfbjdbadhb_testF device.
call blkid on a disk member of this raid tell me this:
/dev/sde: TYPE="isw_raid_member"
and on "mdadm" created raid:
/dev/sdd: UUID="a974b525-993a-1481-f860-6471f3f120e1"
UUID_SUB="eb22aee2-b2ee-e56d-1008-44d52c63564d" LABEL="archipel:0"
TYPE="linux_raid_member"

This misled me because mdadm udev rules uses the output of blkid to
mount raids which have type "isw_raid_member".
What disturbs me is that mdadm cannot mount raid created by dmraid
with type isw_raid_member.

About outputs:
mdadm -I --verbose /dev/sde
mdadm: no RAID superblock on /dev/sde.

Seblu can you verify that:
   export IMSM_NO_PLATFORM=1
   mdadm -E /dev/sde

archipel ~ 0 # export IMSM_NO_PLATFORM=1
archipel ~ 1 # mdadm -E /dev/sde
/dev/sde:
          Magic : Intel Raid ISM Cfg Sig.
        Version : 1.1.00
    Orig Family : 5a8ed623
         Family : 5a8ed623
     Generation : 00000000
           UUID : ae2e9cd8:7fa43248:47c694a1:24990cbc
       Checksum : c23b6c88 correct
    MPB Sectors : 1
          Disks : 2
   RAID Devices : 1

  Disk00 Serial : 66faec8-9f5b237d
          State : active
             Id : 00040000
    Usable Size : 1019486 (497.88 MiB 521.98 MB)

[testF]:
           UUID : 6640a4cc:5faa1ce3:c1bff2b3:1093ca7d
     RAID Level : 1
        Members : 2
          Slots : [UU]
    Failed disk : none
      This Slot : 0
     Array Size : 1014446 (495.42 MiB 519.40 MB)
   Per Dev Size : 1014792 (495.59 MiB 519.57 MB)
  Sector Offset : 0
    Num Stripes : 3963
     Chunk Size : 64 KiB
       Reserved : 0
  Migrate State : idle
      Map State : normal
    Dirty State : clean

  Disk01 Serial : 0b540c6-4e527908
          State : active
             Id : 00050000
    Usable Size : 1019486 (497.88 MiB 521.98 MB)

finds no superblock? It may be that dmraid has laid down something incompatible.

quoted

As has been mentioned elsewhere, mdadm only recognised IMSM arrays on
machines with IMSM hardware.  I'm not entirely happy about this and may well
change it.

I have trouble answering the "least surprise" question in this area.

Is it more surprising to go into your BIOS, explicitly turn off raid
support and still see raid devices showing up?

Or is it more surprising to take a raid array from a raid enabled
system to raid disabled system and wonder why things won't assemble?

For safety I think it is better if mdadm not perform operations that
might be incompatible with the platform option-rom.  But if you need
to recover to a usb attached drive, or some other
platform-incompatible configuration, you can use the environment
variable in a pinch.

--
Dan

-- 
Sébastien Luttringer
www.seblu.net
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