Re: raind-1 resync speed slow down to 50% by the time it finishes

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Tue, Aug 04, 2009 at 03:21:28PM -0700, David Rees wrote:
> 2009/8/1 Keld Jørn Simonsen <keld@dkuug.dk>:
> > On Sat, Aug 01, 2009 at 08:13:45AM -0700, David Rees wrote:
> >> No - you're getting 120 MB/s from one disk and 80MB/s from another.
> >> How that would add up to 230MB/s defies logic...
> >
> > Why only 80 MB/ when reading? reading from both disks with raid10,f2 are done at the
> > beginning of both disks, thus getting about 115 MB/s from both of them.
> >
> > reading in raid10,f2 is restricted to the faster half of the disk, by
> > design.
> >
> > It is different when writing. there both halves, fast and slow, are
> > used.
> 
> As I mentioned earlier, I was having a hard time visualizing the data
> layout.  So here's a simple diagram that shows near/far layout and why
> Keld was right - with a far layout, reads can be isolated to the fast
> half of the disk.
> 
> It also shows how sequential writes (or any other write that spans
> multiple chunks) force the drives to seek half way across the disk for
> each write.
> 
> Near layout, 4 disks, 2 copies:
> a b c d
> 0 0 1 1
> 2 2 3 3
> 4 4 5 5
> 6 6 7 7
> 
> Far layout, 4 disks, 2 copies
> a b c d
> 0 1 2 3
> 4 5 6 7
> 7 0 1 2
> 3 4 5 6

No, it is rather:

Far layout, 4 disks, 2 copies
a b c d
0 1 2 3
4 5 6 7
1 0 3 2
5 4 7 6

Best regards
keld
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