Re: XFS on top RAID10 with odd drives count and 2 near copies

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On 15/02/2012 14:30, Stan Hoeppner wrote:
On 2/15/2012 2:30 AM, Robin Hill wrote:
On Tue Feb 14, 2012 at 05:27:43PM -0600, Stan Hoeppner wrote:

Maybe I simply don't understand this 'magic' of the f2 and far layouts.
  If you only read the "faster half" of a spindle, does this mean writes
go to the slower half?  If that's the case, how can you read data that's
never been written?

Writes go to both halves, as normal for a mirrored setup, which is why

Huh?  A 'normal' RAID setup mirrors one disk to another.  You're
describing data being mirrored from the outer half of a single disk to
the inner half.  Where's the Redundancy in this?  This doesn't make sense.


Like Robin said, and like I said in my earlier post, the second copy is on a different disk.

its write performance is lower than that of a near layout array (more
head movement required). Reads will (normally) come from the faster
(outer) half of the disk though, so read performance is better. In most
cases workloads are read-heavy, so this comes out as a significant gain.

Again, this makes no sense.  You're simply repeating what David said.
Neither of you seem to really understand this, or are simply unable to
explain it correctly, technically.


As far as I can see, you are the only one in this thread who doesn't understand this. I'm not sure where the problem lies, as several people (including me) have given you explanations that seem pretty clear to me. But maybe there is some fundamental point that we are assuming is obvious, but you don't get - hopefully it will suddenly click in place for you.

Have a look at this:

<http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10>

Forget writes for a moment. Forget the second copy on the inner halves of the disks. Then as far as reading is concerned, the raid10,f2 layout looks /exactly/ like a raid0 stripe, using only the outer halves of the disks. That means large reads use all the spindles as they read whole stripes at a time, just like raid0. And because all data is on the outer halves of the disks, the average bandwidth is higher than if it were spread over the whole disk, and the average seek is faster because the head movement is smaller - this is standard "short stroking" speed improvement.

Write performance is lower because it needs more head movement to make the second copy in the inner halves of the disks (as well as the copy to the outer halves). But if you have a reasonable read-to-write ratio, the total performance is higher overall.

Maybe Neil will jump into the fray and answer my original question.


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