Re: RAID 5 data structures
Joe McDonagh wrote:
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?
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
IIRC, the 64k chunks are transparent to the FS. That's why you're still
using 4k blocks. Strip size, width, chunk size, these terms have
different meanings depending on who you talk to. But, if memory serves,
you're seeing this because the FS is at a different level of abstraction
than RAID.
So multiple files can fit into a chunk and when they need access the
array fetches the whole chunk?
Thanks
Sam
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