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If you replaced a broken Harddisk in a RAID 5 the raid must be resynced, which can last lots of hours, if it has some TeraBytes.

If this resync is interrupted by powerloss, can the server be rebooted and the resync be started over again?

(I am using Ubuntu 12.04 on my server in my livingroom and it shut down into hibernate mode, cause I accidently hit the powerbutton while the resync was still running)

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Yes, it can be and even should be — using write intent bitmap which are exactly the things which were brought into as time-savers — man 4 md:

«…

Bitmap Write-intent Logging

From Linux 2.6.13, md supports a bitmap based write-intent log. If configured, the bitmap is used to record which blocks of the array may be out of sync. Before any write request is honoured, md will make sure that the corresponding bit in the log is set. After a period of time with no writes to an area of the array, the corresponding bit will be cleared.

This bitmap is used for two optimisations.

Firstly, after an unclean shutdown, the resync process will consult the bitmap and only resync those blocks that correspond to bits in the bitmap that are set. This can dramatically reduce resync time.

…»

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No, because it can't be sure that something else didn't touch the drives while it was off.

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and if you are sure, it wasn't touched? (my server in my livingroom shut down into hybernate mode, cause I hit the powerbutton while the resync was still running) –  rubo77 Oct 8 '12 at 21:24
    
Nope, and I doubt any developer would add such a feature. That feature would make the code more complicated, and more complicated means potentially more bugs. That cost does not out weigh the benefits for such an infrequent use case. –  longneck Oct 8 '12 at 21:40
    
so what should I do now? is my data lost now? i still didn't switch it on again. –  rubo77 Oct 8 '12 at 21:42
    
What makes you think your data is lost? Please post a new question with the answer on superuser.com (this site is for network professionals administering professional systems, not home servers) –  longneck Oct 8 '12 at 21:48
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But if all you were doing is resyncing, then Turing on should be safe. –  longneck Oct 8 '12 at 21:49
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