To see the RAID controller logs, run this command:
/opt/MegaRAID/MegaCli/MegaCli -AdpEventLog -GetLatest 1000 -f events.log -aALL
The events.log file contained entries like these which indicates a problem with the disk:
Code: 0x0000006e
Class: 0
Locale: 0x02
Event Description: Corrected medium error during recovery on PD 07(e0xfc/s2) at f04cb53
Event Data:
===========
Device ID: 7
Enclosure Index: 252
Slot Number: 2
LBA: 251972435
seqNum: 0x00004f65
Time: Wed Mar 6 05:36:48 2013
Code: 0x00000071
Class: 0
Locale: 0x02
Event Description: Unexpected sense: PD 07(e0xfc/s2) Path 4433221101000000, CDB: 28 00 0f 04 d1 f7 00 01 e0 00, Sense: 3/11/00
Event Data:
===========
Device ID: 7
Enclosure Index: 252
Slot Number: 2
CDB Length: 10
CDB Data:
0028 0000 000f 0004 00d1 00f7 0000 0001 00e0 0000 0000 0000 0000 0000 0000 0000 Sense Length: 18
Sense Data:
00f0 0000 0003 000f 0004 00d2 0046 000a 0000 0000 0000 0000 0011 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
seqNum: 0x00004f64
Time: Wed Mar 6 05:36:43 2013
First off all, see this post: What are the different widely used RAID levels and when should I consider them
Notice the difference between RAID 10 and RAID 01.
Match this to your setups (both of them labeled as RAID 10 in your text). Look carefully.
Read the part in the link I posted where it states:
RAID 01
Good when: never
Bad when: always
I think your choice should be obvious after this.
Edit: Stating things explicitly:
Your first setup is a mirrored pair of 4 drive stripes.
Span 0: 4 drives in RAID0 \
} Mirror from span 0 and span 1
Span 1: 4 drives in RAID0 /
If any drives fails in a stripe then that stripe of lost.
In your case this means:
1 drive lost -> Working in degraded mode.
2 drives lost. Now we use some math.
If the drive fails in the the same span/stripe then you have the same result as 1 drive lost. Still degraded. One of the spans is off-line
If the second drive happens in the other span/stripe:
Whole array off line. Consider your backups. (You did make and test those, right?)
The chance that a second drive fails in the wrong span is 4/7th (4 drives left in the working span, each of which can fail) and only 3/7th that is fails in the span which is already down. Those are not good odds.
Now the other setup, with a stripe of 4 mirrors.
1 drive lost (any of the 4 spans):
Array still works.
2 drives lost:
85% chance that the array is still working.
That is a lot better then in the previous case (which was 4/7th, or 57%).
TL:DR: use the second configuration: It is more robust.
Best Answer
To answer my own question, it is infact possible to migrate live from a single drive raid0 to raid1 (providing an extra disk) with Megaraid cards
The megacli command is as follows (it can also be done from within the WebBios configuration utility of the raid controller):
where [e:s] is the enclosure:slot_number of the extra disk and -l0 is the logical disk ID of the raid0 disk.
Someone can monitor the progress of the reconstruction by issuing the command: