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RAID 5 file structure question.

Posted on 2007-09-27
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I have a question about RAID 5 functionality. Lest suppose that I have a system running 3 hard drives on a RAID 5. Now lets suppose that I want look at the file structure of 1 particular disk only, so I will shutoff the system and take 1 drive out of the RAID system and now I'm going to connect that disk on a different computer with a SAS controller. am'i going to see the hard drive content? what portion of the RAID I'm going to see? All I need is to extract some data from the disk, I don't expect to boot from the disk, I just want to have access to the file structure.
For those who has more experience with RAID 5, please help.

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jdff
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by:garycase
ID: 19975435
There is no "file structure" on a single disk.   The RAID controller spans the logical "disk" across the three drives.   But the file structure is created when you partition and format that "disk" ==>  so the structure information is also spread across the drives.   From Windows' (or whatever OS you're using) perspective, the RAID array is just a disk.   The fact that the disk actually consists of several physical drives isn't even "known" to the OS.
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by:jdff
ID: 19975475
So in another words, if I take that disk just like I said and connect to a controller by it self, I wont be able to see anything? is that right?

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jdff
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by:Lee W, MVP
ID: 19975796
Yes, that's correct.
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by:garycase
ID: 19976187
Sorry for the delayed reply -- went out for the evening -- but as leew confirmed, that's correct.
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by:andyalder
ID: 19977619
If your file is smaller than the stripe size the whole file may be held on a single disk but you would have to search for it with a low level tool. Stripe sizes vary but 128k isn't uncommon and that's quite big for an email for example.
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by:jdff
ID: 19978884
andyalder, do you have an idea of what low level tool can I use for instance?

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jdff
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andyalder earned 1000 total points
ID: 19979548
Only forensic tools or a hex editor like dskprobe.exe, standard tools like getdataback are likely to baulk at the task since they try to reconstruct a filesystem and you don't have a filesystem, just raw data on a disk. You would also have to know what you are looking for, you could for example search the raw disk for "Dear Mr Cooper" and find random scraps of letters to Mr C.
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by:garycase
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ID: 19981318
It's true that you can search the raw data and possibly find some data on a single disk;  but the file structure info is spread across all of the disks on the array, and you simply cannot reconstruct that without having all but one of the disks in the array available.  [you don't need all of them, since the whole idea of the RAID 6 structure is to tolerate the loss of one disk]

IF you have all-but-one disk available, you can reconstruct the data without using the RAID controller with a tool like RAID Reconstructor [http://www.runtime.org/raid.htm ]
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