windows 7 - Make HDD work in single disk mode, which is backup from 2 raid 0 drives

Is using mdadm's RAID-1 of 2 partitions (one on laptop's internal HDD, one on external HDD) a good idea.

I want the system to work as RAID-1 if both drives are present, work as regular volume (degradad RAID-1) if external HDD is unplugged and quickly resync when I plug external HDD again.

Questions:

1. Is it a good idea?
2. Will write-intent bitmap be enough for this task or I need something else?
3. Should I consider doing it at filesystem level (3b. if yes, how?).

Basic requirements are:

1. Quick resync when I re-add the external drive (provided I hasn't changed that partition).
2. More or less consistent data on the removed drive if I remove it not during write/resync operation.

If I remove the drive during resync I expect the data to be somewhat inconsistent, but expect quick resync completion when I re-add it again. E.g. I want the the remaining drive to track what is changed (there can be a lot of changes) and that sync back only those parts that need it.

I would say that this isn't a good idea. All you are protecting against at most is physical/electrical drive problems, and you only ever have one copy of the backup which is very vulnerable while being refreshed.

If a fault occurs on the other drive, or with your machine in general, during the resync you could be left with the main drive being out of action and the "bakcup" drive having a partially updated (i.e. potentially very corrupt) filesystem. This is on of the reasons why most backup solutions include multiple backups (another key reason being that you can have at least one backup offline (and preferably offsite) while updating the another), but is made worse by the fact that the RAID resync is not going to be filesystem aware so will not be able to do anything to ensure a partial resync leaves the filesystem in anything close to a consistent state (for instance it might sync written data before related journal blocks and so on).

If you are RAIDing your whole system in this way then the write-intent write performance hit could be quite significant too (see posts like http://blog.ganneff.de/blog/2008/01/30/write-intent-bitmaps-considere.html), though if you are only RAIDing filesystems that don't see much write activity this may not be a concern for you.

If you do use this technique, make sure you run sync before forcibly ejecting the drive.

Have you considered using rsync to update the backup filesystem instead? This would be slower in your solutions best-case (as a full scan of file sizes and dates is needed even if nothing has changed, which could be significant if the filesystem contains many files) but is usually fast enough and will certainly be faster than a full resync where the RAID driver to decide this is needed.

Looks like external_HDD-based RAID-1 array idea works.

The array starts degraded and tracks changes to it using write-intent bitmap.

When I connect my external HDD, my udev script detects that one of partition suits my array and adds it. It synchronizes (instantly if there were no changes) and then acts as speed-up for read access.

When I disconnect my external HDD (including sudden removal), another udev scripts registers that the drive is removed and the array becomes degraded again.

There is noticeable write performance drop however, but it is acceptable for this usage.

Other advantage is that I can temporarily format that partition for some other use and then bring it back to the array (with a full rebuild).

One problem I see in this, that the filesystem on the removed external HDD might be in a state that cannot be mounted, though modern filesystems with journaling etc do mitigate this risk.

But if you are using mdadm, I can assume you are using some flavour of Linux and would recommend using rsync instead. This would be fast as only changed files would be copied over and it also would allow you to have a script that creates a folder with current date on the external HDD and then create a full backup of your internal harddrive under it but hardlinking the files against the previous backup, thus using space only for the new data. That way you can easily recover a file that might have been deleted on the internal HDD.