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How to read from an offsite disk without splitting a mirrored volume?

(@gerhardmiller)
Active Member Customer

I took one disk (of a 4 mirrored disk setup) offsite and accessed it on another Mac (w/o SoftRAID) via USB, to copy some files from it (without writing back to it).

After plugging it back into my RAID setup this disk then showed as a separate volume with the same name as the original 4 mirrored disk setup, with a status "degraded - 3 missing secondary disks". Looks like maybe Mac OS wrote something on the disk while connected to this other Mac (obviously without me consciously causing this; also, while connected to this other Mac, the disk spontaneously disconnected once, maybe that caused some "change" to the disk).

Having read through some of the other postings I will now remove that secondary disk from my original setup, initialize it, add it back as a secondary disk, and let it perform a full rebuild. Not great (given that in my view nothing really changed on the secondary disk), but I'll do it.

Now my question: What is the recommended procedure to read from a mirrored secondary disk (not the primary disk, which remains in the raid setup) on another Mac (without SoftRAID) without causing the mirrored volume to split into two later when plugging the secondary disk back into the raid setup?

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Topic starter Posted : 21/08/2018 3:49 pm
(@softraid-support)
Member Admin

This is expected behavior.

When a secondary disk is connected, if the primary is not available, it is converted to a primary with a new VolumeID. This is so when the primary shows up later, it cannot be rebuilt. (SoftRAID cannot know which should be primary)

So there are two ways to manage this:
If you move a disk to another computer, move the primary.
Or, split mirror, read only secondary disk, then you can move that disk to a different computer to copy files off of it, and "Convert read only secondary disk" when you get back to let it rebuild into the volume.

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Posted : 21/08/2018 5:20 pm
(@gerhardmiller)
Active Member Customer

Thank you for your quick reply!

split mirror, read only secondary disk, then you can move that disk to a different computer to copy files off of it, and "Convert read only secondary disk" when you get back to let it rebuild into the volume.

This is useful and should indeed work for me.

When a secondary disk is connected, if the primary is not available, it is converted to a primary with a new VolumeID. This is so when the primary shows up later, it cannot be rebuilt. (SoftRAID cannot know which should be primary)

Not sure I understand this fully. Let me clarify my setup:

1) MacMini Server with SoftRAID: 4 disk mirrored raid, with one primary and three secondary disks.

2) Offsite Macbook Pro without SoftRAID: This is where I am temporarily connecting one of the secondary disks to, to copy some files off it.

When the secondary disk is connected to 1), there is always a primary disk, so here the secondary disk should not be converted to a primary disk.

When the secondary disk is connected to 2), there is no SoftRAID driver involved. So I'd assume that here the secondary disk is just accessed by Mac OS, and no secondary/primary change by SoftRAID can take place.

Hence I wonder at what stage (and why) the secondary disk is converted to a primary disk. I'd guess this happens when I plug the disk back again into 1), then SoftRAID finds out that the disk has been written to (by whatever Mac OS process, indexing the disk's contents or whatever, without SoftRAID involved) and can then not determine anymore reliably what the state of the disk is?

If so, I wonder whether an alternative to the "split to read-only secondary" approach might be to mount the secondary disk in 2) read-only (assuming this is possible on Mac OS). Benefit would be that I don't need to upfront carry out the "split" operation each time I move a secondary disk offsite (for the odd chance that I need to read from it). Any thoughts on / experience with this "read-only mounting"?

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Topic starter Posted : 21/08/2018 5:53 pm
(@softraid-support)
Member Admin

My description is accurate, but I will give some additional details:

There is a (limited version) SoftRAID driver on EVERY OS X install. That is how your volume mounts.

When you connect any SoftRAID mirror disk to a system, if it is a primary, it mounts. If a secondary, the driver first waits to see if a primary is available, before converting it to a primary with a new VolumeID.

Here is what you are missing: A Secondary disk is NOT a mountable volume. It cannot mount. In order to mount, it must be converted to a primary first. This is why when you connect a secondary to another computer and bring it back, it cannot rebuild. A disk driver cannot "figure out" programatically whether it was supposed to be a new master, or resume being a secondary. That is why it cannot rebuild, they are now two separate volumes.

A "read only secondary disk" is a "new volume", but the disk is locked, so SoftRAID can compare it to the original primary and add it back at some point in the future, without requiring a complete rebuild.

Hope this helps.

If you are going to frequently move a disk, unmount your volume, move the primary, copy the data over ,then bring it back.

Or move the entire enclosure. Then there are no issues. (A mirror cannot rebuild, without the full SoftRAID driver, but this is a temporary move so you can copy data off, as you describe it)

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Posted : 21/08/2018 6:34 pm
(@gerhardmiller)
Active Member Customer

Thank you for your prompt response and for the very detailed explanation, now this all makes sense. And I have a solution with the read-only secondary split method. :-)

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Topic starter Posted : 22/08/2018 2:51 am
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