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Re: RAID 5 data structures

On Wed, Mar 11, 2009 at 07:11:11AM -0500, Sam Leon wrote:
> I have a funny question.  I have been playing with a 3 disk raid 5 setup  
> for my desktop.  I guess I don't fully understand how the "stripe" is  
> managed or even what it is.  I know the stripe is made up of a chunk  
> from each disk.  Now I always thought of the stripe in raid the same as  
> a block in ext3 or a cluster in ntfs.  Meaning if I have a 1k file that  
> I write to an ext3 filesystem with 4k blocks, my 1 k file will take up  
> one block thus wasting 3k of space.
> Now I thought the stripe in raid followed the same principle.  Meaning  
> if I have a 3 disk array with 64k chunks then my data stripe is the  
> number of disks minus one drive because of parity so in this 3 disk  
> array I would have a data stripe of 128k.  So how much of that space  
> would my 1k file take up?  Would it take up the whole 128k stripe or  
> just one chunk leaving the other chunk free for something else?

I am no expert at this, but my understanding is

the chunk size has to do with the data / parity blocks.  The raid system
will work in 64K chunks, so your 1K file will take up 1k of data and
whatever of fs space.  but the raid device will have to read write 64K
chunks of information from each disk. so for 3 disks it is 2 x 64k of
data + 1 64K of parity.

> When I migrated my root drive over to the raid5 array I made, I was  
> afraid that it would use alot more space on the raid array since it is  
> full of very small files but to my surprise df -h reported the same  
> values for / on the array and / on the single disk that I had copied the  
> data from.  So what is going on here?
> Thanks,
> Sam
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