swidth in RAID

[aurfalien]

On Jun 30, 2013, at 6:38 PM, Dave Chinner wrote:

> On Sun, Jun 30, 2013 at 04:42:06PM -0500, Stan Hoeppner wrote:
>> On 6/30/2013 1:43 PM, aurfalien wrote:
>> 
>>> I understand swidth should = #data disks.
>> 
>> No.  "swidth" is a byte value specifying the number of 512 byte blocks
>> in the data stripe.
>> 
>> "sw" is #data disks.
>> 
>>> And the docs say for RAID 6 of 8 disks, that means 6.
>>> 
>>> But parity is distributed and you actually have 8 disks/spindles working for you and a bit of parity on each.
>>> 
>>> So shouldn't swidth equal disks in raid when its concerning distributed parity raid?
>> 
>> No.  Lets try visual aids.
>> 
>> Set 8 coffee cups (disk drives) on a table.  Grab a bag of m&m's.
>> Separate 24 blues (data) and 8 reds (parity).
>> 
>> Drop a blue m&m in cups 1-6 and a red into 7-8.  You just wrote one RAID
>> stripe.  Now drop a blue into cups 3-8 and a red in 1-2.  Your second
>> write, this time rotating two cups (drives) to the right.  Now drop
>> blues into 5-2 and reds into 3-4.  You've written your third stripe,
>> rotating by two cups (disks) again.
>> 
>> This is pretty much how RAID6 works.  Each time we wrote we dropped 8
>> m&m's into 8 cups, 6 blue (data chunks) and 2 red (parity chunks).
>> Every RAID stripe you write will be constructed of 6 blues and 2 reds.
> 
> Right, that's how they are constructed, but not all RAID distributes
> parity across different disks in the array. Some are symmetric, some
> are asymmetric, some rotate right, some rotate left, and some use
> statistical algorithms to give an overall distribution without being
> able to predict where a specific parity block might lie within a
> stripe...
> 
> And at the other end of the scale, isochronous RAID arrays tend to
> have dedicated parity disks so that data read and write behaviour is
> deterministic and therefore predictable from a high level....
> 
> So, assuming that a RAID5/6 device has a specific data layout (be it
> distributed or fixed) at the filesystem level is just a bad idea. We
> simply don't know. Even if we did, the only thing we can optimise is
> the thing that is common between all RAID5/6 devices - writing full
> stripe widths is the most optimal method of writing to them....

Am I interpreting this to say;

16 disks is sw=16 regardless of parity?

As the thing common is number of disks.   Or 1 parity as the least common denom which would mean sw=15?

Peter brought this up;

The main goal is trying to the reduce the probability of
read-modify-write.

Which is a way for me to think it as  "don't over subscribe".

- aurf