[UPDATED] Setting up RAID 1 consisting of ramdisk and /dev/sda2

I'm not quite sure which board this question belongs to, so I'll post it in the newbie section, as I feel like one right now.
I need help with setting up my root partition as a RAID 1 of a ramdisk and a partition on my disk.
I have a very fresh arch install (2014-03-20) running on a this system:
512M /dev/sda1 vfat -> /boot (boot- and uefi-partition)
12G /dev/sda2 ext4 -> /
104.5G /home
64G RAM
My wish is to run / as a RAID 1 of the 12G /dev/sda2 ext4 and a 12G Ramdisk ext4.
I have read this great post on doing exactly this and the entire thread it is posted in, concerning running Arch entirely in RAM. But, to the best of my understanding, the use of a simple block device in RAM such as /dev/rd/0 has since been deprecated. I have been able to set them up manually with:
$ sudo mknod -m 660 /dev/rd b 1 1
But when I try to zero out the space I need on the device I'm unable to write more than 16 Mb of data to the device.
$ sudo dd if=/dev/zero of=/dev/rd bs=1K count=12288k
dd: error writing ‘/dev/rd’: No space left on device
16385+0 records in
16384+0 records out
16777216 bytes (17 MB) copied, 0.0222693 s, 753 MB/s
Likewise, I'm unable to set up a ext4fs larger than 16M on the device.
From what I have understood, this 16 M limit comes from the ramdisk_size kernel param during boot or from the rd_size param when loading the rd kernel module. Only problem is, the kernel param ramdisk_size=12582912 doesn't seem to help me and there doesn't seem to be any rd module in the arch kernel.
So my questions for now:
* Is there some other way than rd_size and ramdisk_size to set maximum ramdisk size to 12G?
* Am I doing something wrong?
Last edited by Hrafnahnef (2014-04-01 14:07:57)

This may not be what you want, but I managed to mount a ramdisk as the root /.
I was not interested to tried setting up such an array, as it seems to me that the ramdisk method is more flexible than the array method. By flexibility, I mean that the size of the ramdisk can be changed between reboots, but the array cannot -- I don't think that an array can be expanded or shrinked once created. Also, the disk is not needed to be accessed once the ramdisk is ready; and you can choose what folders of the root hierarchy to be copied into RAM.

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I just wanted to share this information with the iCloud community.
I searched a bit and did not find much information that was useful with regard to my software RAID issue.
I have 27 inch Mid 2011 iMac with SSD and Hard drive which has been great.
I added an external hard drive (I think if I mention any brand name the moderator will delete this post) which includes an nice aluminum case with two 3 TB hard drives within it, and it has a big blue light on the front and is connected via Thunderbolt. This unit is about 2 years old and I have it configured in a 3 TB mirrored RAID (RAID 1) via a software RAID configured via Mac OS Disk Utility.
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Each of the two RAIDs are set with the option of “Automatically rebuild RAID mirror sets” checked.
I put only about 400 gb on the new RAID to let it sit for a ‘burning in period.’
A few days ago the monitoring software from the vendor who sells the aluminum block of drives told me I had a problem. One of the drives had “Failed.”   The monitoring software strangely enough does not distinguish the drives so you can figure out which pair had the issue, so I assumed it was the New 8 TB model. Long story short, it was the older 6 TB model, but that does not matter for this discussion.
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“This is an indication that the Disk Utility application in Mac had a momentary problem communicating with the drive mechanism. As a result, it marked that drive as "failed" in the header information. Unfortunately, once this designation is applied to a drive by the OS, the Disk Utility will thereafter refuse to attempt any further operations with that disk until the incorrect "failed" marker is manually cleared off the drive.”
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“The solution is to remove the corrupted volume header, and allow the generation of a new one….This command will need to be done for each disk in the array… (using Terminal)…
diskutil zerodisk (identifier)
…3. After everything is finished, you should be able to exit Terminal, and go back into the Disk Utility Application to re-configure the RAID array on the device.”
Furthermore they said.
“If the Disk Utility has placed a flag into the RAID array header (which exists on both drives) then performing this procedure on a single drive will not correct anything.”
And…
“When a drive actually does fail, it typically stops appearing in the Disk Utility application altogether. In that circumstance, it will never be marked "failed" by the Disk Utility, so the header erase operation is not needed.”
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First I noted that the OS Disk Utility did NOT show a fault in the RAID. It listed both RAIDS as “Online.’ Thus no rebuilding was needed and it did not begin the rebuild process.
The Vendors disk monitor software saw some fault, but Mac was still able to read and write to the RAID, both disks in the mirror. I wrote a folder to the RAID and with various rebooting steps I pulled the “Bad” drive and looked at the “Good” Drive….the folder was there…I put the Bad drive back in and pulled the Good Drive and the folder was there on the “bad” drive. So it wrote to both drives. AND THE VENDORS MONITORING SOFTWARE SHOWED THE PREVIOUSLY LABELED ‘BAD’ DRIVE AS ‘GOOD’ AND THE MISSING DRIVE SLOT AS ‘BAD’.
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This process took a few hours as best I can tell (let it run overnight) and the next day the RAID was fine and the Vendors RAID monitor did not show a fault any longer.
So, the Vendors RAID monitoring software reporting a “FAILED” drive without any specific error codes to look up. Perhaps they could have more info for the user on the specific fault? The support line of the the Vendor said with certainty “the Volume Header is corrupted” and THE ONLY FIX is to completely ZERO THE DRIVE! This was not necessary as it turns out.
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“I've also sometimes seen the drives get marked as "failed" by the disk utility due to a shaky connection. In some cases, swapping the ends of the Thunderbolt cable will help with this. Something to try, perhaps, if your problems come back. “
Ya Right…..
Mike

Follow up.
After going through the Zeroing process and rebuilding the RAID set three times, with various configurations, LaCie finally agreed to repair the unit under warrantee.
I tried swapping the power supplies and thunderbolt wires, tried taking the drive out of series with the newer big brother of it. And it still failed after a few days.
I just wanted to share more of what I learned with regard to rebuilding the RAID sets via the Terminal. The commands can be typed partially and a help paragraph will come up to give VERY cryptic descriptions of the proper use of the commands.
First Under terminal you can used the command "diskutil appleRAID list" to list those drives which are in the RAID. This gives you the ID number for each physical drive. For example:
AppleRAID sets (1 found)
===============================================================================
Name:                 LaCie RAID 3TB
Unique ID:            84A93ADF-A7CA-4E5A-B8AE-8B4A8A6960CA
Type:                 Mirror
Status:               Online
Size:                 3.0 TB (3000248991744 Bytes)
Rebuild:              manual
Device Node:          disk4
# DevNode   UUID                                  Status     Size
0 disk3s2   D53F6A81-89F1-4FB3-86A9-8808006683C2 Online     3000248991744
- disk2s2   E58CA8F5-1D2C-423A-B4BE-FBAA80F85879 Spare      3000248991744
===============================================================================
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The command is "diskutil appleRAID remove" and the cryptic help paragraph says:
Usage: diskutil appleRAID remove MemberDeviceName|MemberUUID
        RAIDSetVolumePath|RAIDSetDeviceName|RAIDSetUUID
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Started RAID operation on disk4 LaCie RAID 3TB
Removing disk from RAID
Changing the disk type
Can't resize the file system on the disk "disk2s2"
Error: -69827: The partition cannot be resized
But I was able to remove it using the graphical interface Disk Utility program using the delete key.
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Michaels-iMac:~ mike_aronis$ diskutil appleraid update autorebuild 1 /dev/disk4
Error updating RAID: Couldn't modify RAID (-69848)
Michaels-iMac:~ mike_aronis$
In my haste to rebuild the RAID set for the third or forth time as LaCie led me through the testing this and test that phase, I forgot to click the "Auto Rebuild" option in the Disk Utility program.
Question for the more experienced:
As I was working on this issue, I notice that each time I rebooted and did work in the Terminal (with and without the RAID plugged in to the thunderbolt connection) I notice that the list of drives would change and my main boot drive would not stay listed as drive 0! Some times it would be drive 0, sometimes the RAID would be listed as Drive 0. It's strange to me...I would have thought the designation for Drive0 and Drive1 would always be my two build in drives (SSD and spinning drive).
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Please read this whole message before doing anything.
This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.
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Hmm.. This does explain a few things, especially the part about having X.5.2 as a minimum.. The slight problem with the the disk that was packaged with the machine is that my dad (for whom I'm setting it up) has been "cleaning up" his disks - I've had a hard time finding the Leopard disk for that matter... - and is now on a trip for the next two weeks. I'm now configuring a new external install - entirely fresh and updated to X.5.4 - to try booting from that.
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so your problem is TWICE as important, and you start another thread.
https://forum-en.msi.com/index.php?topic=104173.0

[UPDATED] Setting up RAID 1 consisting of ramdisk and /dev/sda2

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