Re: [LSF/MM TOPIC] De-clustered RAID with MD

Johannes Thumshirn
2018-01-31T10:58:37Z

David Brown <david.brown@hesbynett.no> writes:
> That sounds smart. I don't see that you need anything particularly
> complicated for how you distribute your data and parity drives across
> the 100 disks - you just need a fairly even spread.

Exactly.

> I would be more concerned with how you could deal with resizing such an
> array. In particular, I think it is not unlikely that someone with a
> 100 drive array will one day want to add another bank of 24 disks (or
> whatever fits in a cabinet). Making that work nicely would, I believe,
> be more important than making sure the rebuild load distribution is
> balanced evenly across 99 drives.

I don't think rebuilding is such a big deal, lets consider the following
hypothetical scenario:

6 Disks with 4 data blocks (3 replicas per block, could be RAID1 like
duplicates or RAID5 like data + parity, doesn't matter at all for this
example)

D1 D2 D3 D4 D5 D6
[A] [B] [C] [ ] [ ] [ ]
[ ] [ ] [ ] [A] [D] [B]
[ ] [A] [B] [ ] [C] [ ]
[C] [ ] [ ] [D] [ ] [D]

Now we're adding one disk and rebalance:

D1 D2 D3 D4 D5 D6 D7
[A] [B] [C] [ ] [ ] [ ] [A]
[ ] [ ] [ ] [ ] [D] [B] [ ]
[ ] [A] [B] [ ] [ ] [ ] [C]
[C] [ ] [ ] [D] [ ] [D] [ ]

This moved the "A" from D4 and the "C" from D5 to D7. The whole
rebalancing affected only 3 disks (read from D4 and D5 write to D7).

> I would also be interested in how the data and parities are distributed
> across cabinets and disk controllers. When you manually build from
> smaller raid sets, you can ensure that in set the data disks and the
> parity are all in different cabinets - that way if an entire cabinet
> goes up in smoke, you have lost one drive from each set, and your data
> is still there. With a pseudo random layout, you have lost that. (I
> don't know how often entire cabinets of disks die, but I once lost both
> disks of a raid1 mirror when the disk controller card died.)

Well this is something CRSUH takes care of. As I said earlier it's a
weighted decision tree. One of the weights could be to evenly spread all
blocks across two cabinets.

Taking this into account would require a non-trivial user interface and
I'm not sure if the benefits of this outnumber the costs (at least for
an initial implementation).

Byte,
Johannes
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Johannes Thumshirn Storage
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