Re: [Evms-devel] Possible to create a degraded RAID5 array with EVMS?

[James L. <jam...@ho...>]

Thanks for the reply Steve.

I've retried setting up a degraded RAID5 array as described below, and have 
hit a different error in EVMS ("*** glibc detected *** free(): invalid next 
size (fast): 0x0819d720 ***").  Procedure was:


1)  Delete all md arrays and all segments from /dev/sdb (using EVMS).  
Worked fine.

2)  Close and reopen EVMS to check all is still well.  It is.  Close it down 
again.

3)  Using cfdisk, create two logical (not primary) partitions on /dev/sdb 
(which had just free space), 100MB each (these are /dev/sdb5 and /dev/sdb6). 
  Mark them as type 0xfd (Linux RAID autodetect).  Save changes and again 
reload and close EVMS to check it's still happy; which it is.

4)  Use mdadm to create a degraded RAID5 array using the following command:
"mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb5 /dev/sdb6 
missing"

5)  mdadm report that this array gets created successfully (in a degraded 
state):
james@ubuntu-fileserver:~$ sudo mdadm --detail /dev/md0
/dev/md0:
        Version : 00.90.01
  Creation Time : Wed Nov 16 23:05:41 2005
     Raid Level : raid5
     Array Size : 192512 (188.00 MiB 197.13 MB)
    Device Size : 96256 (94.00 MiB 98.57 MB)
   Raid Devices : 3
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Wed Nov 16 23:05:41 2005
          State : clean, degraded
Active Devices : 2
Working Devices : 2
Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           UUID : 6f0fa83a:ae0b4a99:a199ce36:9e21815c
         Events : 0.4

    Number   Major   Minor   RaidDevice State
       0       8       21        0      active sync   /dev/.static/dev/sdb5
       1       8       22        1      active sync   /dev/.static/dev/sdb6
       2       0        0        -      removed

6)  Start EVMS GUI.  Gives me a warning about /md/md0 being in a degraded 
state (which is fine).  It doesn't give me the option to create an EVMS 
volume from the /md/md0 array (which is not good).  It sees the array, and 
reports it as both degraded and corrupted (it shouldn't be, right?).

7)  Close down EVMS and restart it.  Again, I get a warning about the array 
being degraded.  Now I do have the option to create a volume from the array. 
  On trying to do this however, I get the following error message:
*** glibc detected *** free(): invalid next size (fast): 0x0819d720 ***
and EVMS hangs completely.

I've attached a gzipped copy of the evms-engine.log file.

Hope this helps...

What is the relationship between the EVMS MD plugin and mdadm by the way?  
Do they use the same code, or are they entirely different programs that 
interface in their own way with the Linux RAID driver?  Should arrays 
created by mdadm and array created by EVMS be identical once they have been 
created?

James




>From: Steve Dobbelstein <st...@us...>
>To: "James Lee" <jam...@ho...>
>CC: evm...@li...
>Subject: Re: [Evms-devel] Possible to create a degraded RAID5 array with 
>EVMS?
>Date: Wed, 16 Nov 2005 11:53:51 -0600
>
>"James Lee" <jam...@ho...> wrote on 11/14/2005 06:04:26 PM:
>
> > Hi there,
>
>Hi, James.
>
> > I'm setting up a RAID5 fileserver on a LAN, and using EVMS to manage the
> > RAID array.
> >
> > Due to having to shift data around from a current array to a new one,
>I'll
> > be needing to (temporarily) create a RAID5 array in a degraded state. In
> > other words create an 3-drive RAID5 array, but with one drive missing
>(i.e.
> > using only 2 drives).
>
>EVMS does not currently support building a degraded array.  One of the
>design principles behind EVMS is that it should only build "safe"
>configurations.  A degraded array is subject to a single disk failure,
>which defeats the purpose of building the RAID5.
>
> > Now this is possible with mdadm by passing in one of the drives in the
>array
> > as "missing" rather than the actual drive name. In EVMS though, there
> > doesn't seem to be an option to do this and it's not possible to create 
>a
>
> > RAID5 array with fewer than three drives.
>
>Right.  And our current recommendation for building a degraded array is to
>use mdadm, which you did.
>
> > Now I tried to just use mdadm to create the RAID5 array and then use 
>EVMS
>
> > afterwards. This caused EVMS to start failing with the following error:
> >
> > *** glibc detected *** corrupted double-linked list: 0x08065a30
> >
> > This rendered the system unbootable until I wiped the offending array.
>
>Yikes.  That is not supposed to happen.  Can you run with the debug level
>set to debug (e.g., "evmsgui -d debug") and then send me the gzipped
>/ver/log/evms-engine.log.  Hopefully it will have some clues.
>
> > So is there any way to create degraded arrays from within EVMS?
>
>At the moment, no.  However, we have been getting several requests for this
>functionality.  Due to customer demand we will revisit our "safe" design.
>I think allowing the creation of a degraded array with a warning message
>should be sufficient.
>
>Steve D.
>