Create software raid5 with one drive offline.

[Gherald]

I am wondering if it is possible in theory/practice to create a software raid 5 with one of the drives being offline from the get-go.

Specifically, I want to create a raid5 with eight 200gb drives, but one them already has information that I intend to keep. So I want to initialize the 8-drive raid with only 7 drives (essentially a regular 8-drive raid with 1 offline), then bring the 8th drive online once I have copied the files over to the raid.

Is this feasible?

[evan18h]

I did a quick search on google, becuase I was particularly interested in knowing myself.. and what I got was that it is possible depending on what you are using (I think its easier with a hardware RAID over a software one), I've never attempted this so I would'nt know for sure... but I would like to know what your using to build this.

[futile1]

This can be easily done. From the mdadm man page:

CREATE MODE
Usage: mdadm --create device --chunk=X --level=Y
--raid-devices=Z devices


This usage will initialise a new md array, associate some devices with it, and activate the array.

This the --auto option is given (as described in more detail in the section on Assemble mode), then the md device will be created with
a suitable device number if necessary.

As devices are added, they are checked to see if they contain raid superblocks or filesystems. They are also checked to see if the
variance in device size exceeds 1%.

If any discrepancy is found, the array will not automatically be run, though the presence of a --run can override this caution.

To create a "degraded" array in which some devices are missing, simply give the word "missing" in place of a device name. This will
cause mdadm to leave the corresponding slot in the array empty. For a RAID4 or RAID5 array at most one slot can be "missing"; for a
RAID6 array at most two slots. For a RAID1 array, only one real device needs to be given. All of the others can be "missing".

When creating a RAID5 array, mdadm will automatically create a degraded array with an extra spare drive. This is because building the
spare into a degraded array is in general faster than resyncing the parity on a non-degraded, but not clean, array. This feature can
be over-ridden with the --force option.
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[Gherald]

Hmm, after reading up on raidtools' "raidreconf" and "lsraid" I think I am going to go with a smaller 4-5 drive raid and add drives later as needed.

To dispell some of the risk, I think I will make a two ~100gb partitions on each drive and have two seperate logical Raids. This way I can expand each half independently and move all crucial data back and forth to the one that is not currently being expanded

[fmalabre]

What command do you use to start your raid with one missing drive?
I'm going crazy, my raid won't start, i've been tring for hours...
Couldn't find anything on google...
lsraid says my 2 drives are ok...