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reduce RAID5 to RAID0

Posted on 2012-03-19
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Last Modified: 2012-06-27
Hi,

I have a HP Proliant DL360 G6 server, with one controller. At the moment I have four disks in a RAID5 array. Is it possible to "reduce" this RAID5 array to a RAID0 array with only two disks? Without losing any data? (The remaining two disks will need to be configured as a RAID1 array.)

regards,

Paul
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Question by:IT_Assist
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by:dlethe
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Not possible, the math doesn't work out.

a 4-disk RAID5 has effectively 3 disks worth of data once you remove the XOR/redundancy.  If you go to a RAID0, then you need 3 disks .
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by:IT_Assist
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Ah, what a pity. I thought already so, but you never know if some genius invented a tool to reduce those three disks to two. Technically speaken, there is enough diskspace on two disks, to contain the data spread over three.
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andyalder earned 500 total points
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You can almost do it, the SAAP license enables shrinking an array, but I don't think it allows you to shrink a logical disk. If you have two logical disks striped over the array of physical disks you could delete one and have space.

Easier to buy a second disk backplane for 4 more disk bays, probably cheaper too. That would lose the CD ROM though, there isn't room for that when you put 8 SFF hard disks in a 1U server.
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by:dlethe
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Well, then, IT_Assist,  If you can delete 3 disks to 2, then why not go the extra mile and repeat the process and reduce 2 to 1, and continue.

(Actually you can technically reduce all that data to just one bit with data compression, where 1=the pattern on those 3 disks, and anything starting with 0 represents anything else.  But point here is that there is no free lunch in this universe. ;)
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by:andyalder
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You can almost do that too David; but it involves first deleting a load of old files, then shrinking the space used by the filesystem and defragging, then shrinking the logical disk, then re-leveling and taking the physical disks out of the array.

Shrinking the logical disk is child's play for the controller but it isn't enabled in the GUI probably because it's too scary to contemplate what would happen if someone shrunk it without shuffling the remaining data to the left first. The controller is clever, but not clever enough to understand the whole filesystem format. Still, they could check that the OS partition shuffling had been done first by only shrinking it by removing stripes that were zeroed out.
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by:andyalder
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"That" was not addressing "Actually you can technically reduce all that data to just one bit with data compression" which is clearly ludicrous. It was addressing your first paragraph.
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by:dlethe
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Ludicrous that you could compress into a single bit? Granted if you never studied discrete math or got an advanced degree in CS then it might not be obvious, but no need to get insulting because you don't understand.
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by:andyalder
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by:IT_Assist
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It's allright, andyalder, I knew dlethe was having a little laugh. :-) Imagine reducing all those gigantic SANs all over the world being reduced to one bit. Imagine losing that one bit due to a disk crash. :-D

Now I thought of the new backplane myself of course, but I just wanted to explore this little idea too. Several people always know more then one. Thanks for your insights.
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by:IT_Assist
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@ andyalder : there is no backplane for only two more disks? It would be nice to keep the cd-rom drive.
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by:dlethe
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Guys, data compression 101.  If you are free to choose the algorithm, you can reduce ANY dataset to just one bit.

So if you have ABCDEF, your data compression algorithm is
1. If data is ABCDEF, then result = 1, and exit
else
  print 0, then if next byte is A print 001, if B 010, if C 011, if D 0001, ...  and so on.   You use a variable-length decoding pattern, and make the first test for the 1 pattern you are looking for.

You could just as easily make 1 represent the entire contents of the library of congress.   But the statement I made is accurate, and if you ever write data compression software, then it is actually quite sound. The trick to compress data is analyzing the data and producing a proper algorithm that can do the most amount of compression and balancing CPU overhead.

You have to have a 0 in front of the pattern to show that it is not the 3-disk compressed data, and then anything after the leading 0 is the rest of a variable-length pattern that uses any other algorithm.  If 1 is in front, and it is the first bit, then that means you have the compressed version of whatever you wanted.

The prof made quite an impression on this exercise, because it served to teach us how data compression really works (and some cryptography techniques, which I spared you), and not to get hung up by thinking like a human.   It all comes down to choosing the right algorithm.
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by:andyalder
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No two disk bakplane I'm afraid, just four.
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by:IT_Assist
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other question (though I don't know yet if I need to replace the existing 4-disk-backplane with an 8-disk-backplane, or just add another 4-disk-backplane) : If I need to replace the existing 4-disk-backplane with an 8-disk-backplane, will the "old" RAID5 automatically be recognized when I plug those disks in? (I suppose it is a yes?)
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by:andyalder
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It's a second backplane, comes with all the cables and screws needed, 516966-B21 (http://www.equanet.co.uk/catalogue/item/Y876528M)

It might be possible to not fit the second cage and keep the DVD, I'll look next time I get one in.
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by:andyalder
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99.9% sure it's not possible to retain the DVD since the backplane would get on the way of seating it.

BTW, I played about with SAAP, shrink array works as expected but there's no shrink logical disk option.
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by:IT_Assist
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okay, nice, very nice ! I'll buy the second backplane, plug it in, restart the server and off we go ! Thanks!
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by:IT_Assist
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Very complete first reply and meaningfull and quick responses to my other subquestions.
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