PERFORCE change 123662 for review

[Ulf Lilleengen]

On Mon, Jul 23, 2007 at 12:36:51PM +0200, Pawel Jakub Dawidek wrote:
> On Fri, Jul 20, 2007 at 03:07:16PM +0200, Alexander Leidinger wrote:
> > Quoting Ulf Lilleengen <lulf at FreeBSD.org> (Fri, 20 Jul 2007 14:35:24 +0200):
> > 
> > [growing RAID-5]
> > > Well, what I do is to attach/create the new subdisk as usual, but since it's a
> > > RAID-5 array that I know is operational, I give the subdisk a flag, and sets the
> > > plex in a resize state. Then, In the raid-5 code, I modify gv_raid5_offset
> > > (which basically computes offsets within a subdisk based on the number of
> > > subdisks and stripesize). However, what I do, is that instead of taking all
> > > subdisks in the calculation, I only take those who does not have the GROW flag
> > > (when reading), and I take all subdisks into calculation when it's a write.
> > > 
> > > This means, that if a create a gv_grow_plex function that reads (stripesize x
> > > sdcount) bytes (from the subdisks that do not have the GROW flag), and writes
> > > that data to the plex (including all subdisks). This way, i sort of overwrite
> > > the old data, but the data is spread out over the new subdisks. I'm sorry if
> > > this might seem a bit complex, but just ask more questions if you didn't
> > > understand.
> > 
> > Do you use the additional drive(s) only to write checksums to them, or
> > do you write real data to it? If the later, how do you make sure you
> > read the right data in case you read data again, which was just written
> > there a moment before (how do you know to read from all subdisks and
> > not only from a subset in this case)?
> 
> You only need to move offset while you synchronize new disk.
> 
> When you start you have:
> 
> 	Disk0	Disk1	Disk2	NewDisk
> 
> 	D0	D1	P0	U
> 	D2	P1	D3	U
> 	P2	D4	D5	U
> 	D6	D7	P3	U
> 	D8	P4	D9	U
> 	P5	D10	D11	U
> 
> After some time you have:
> 
> 	Disk0	Disk1	Disk2	NewDisk
> 
> 	D0	D1	D2	NP0
> 	D3	D4	NP1	D5
> 	U	U	U	U
> -->	D6	D7	P3	U
> 	D8	P4	D9	U
> 	P5	D10	D11	U
> 
> And at the end you have:
> 
> 	Disk0	Disk1	Disk2	NewDisk
> 
> 	D0	D1	D2	NP0
> 	D3	D4	NP1	D5
> 	D6	NP2	D7	D8
> 	NP3	D9	D10	D11
> 	U	U	U	U
> 	U	U	U	U
> 
> Where:
> D<x> - data block
> P<x> - parity block
> NP<x> - new parity block
> U - unused
> --> - if offset in I/O request is below that point, you use four disks,
>       if it is above that point you use only three disks
EXACTLY! 

You're 100 times better than me to explain how it works :) Thanks.

-- 
Ulf Lilleengen