Re: Implementing Global Parity Codes

[Wols Lists <antlists@youngman.org.uk>]

On 29/01/18 10:22, David Brown wrote:
>> I've updated a page on the wiki, because it's come up in other
>> discussions as well, but it seems to me if you need extra parity, you
>> really ought to be going for raid-60. Take a look ...
>>
>> https://raid.wiki.kernel.org/index.php/What_is_RAID_and_why_should_you_want_it%3F#Which_raid_is_for_me.3F
>>
>>
>> and if anyone else wants to comment, too? ...
>>
> 
> Here are a few random comments:
> 
> Raid-10-far2 can be /faster/ than Raid0 on the same number of HDs, for
> read-only performance.  This is because the data for both stripes will
> be read from the first half of the disks - the outside half.  On many
> disks this gives higher read speeds, since the same angular rotation
> speed has higher linear velocity at the disk heads.  It also gives
> shorter seek times as the head does not have to move as far in or out to
> cover the whole range.  For SSDs, the layout for Raid-10 makes almost no
> difference (but it is still faster than plain Raid-1 for streamed reads).

Except that most drives don't do that nowadays, they do "constant linear
velocity" so the drive speeds up or slows down depending on where the
heads are, I believe.
> 
> For two drives, Raid-10 is a fine choice on read-heavy or streaming
> applications.

Which is just raid-1, no?
> 
> I think you could emphasise that there is little point in having Raid-5
> plus a spare - Raid-6 is better in every way.

Agreed. I don't agree raid-6 is better in *every* way - it wastes space
- but yes once you have enough drives you should go raid-6 :-)
> 
> You should make a clearer distinction that by "Raid-6+0" you mean a
> Raid-0 stripe of Raid-6 sets, rather than a Raid-6 set of Raid-0 stripes.
> 
Done.

> There are also many, many other ways to organise multi-layer raids.
> Striping at the high level (like Raid-6+0) makes sense only if you have
> massive streaming operations for single files, and massive bandwidth -
> it is poorer for operations involving a large number of parallel
> accesses.  A common arrangement for big arrays is a linear concatenation
> of Raid-1 pairs (or Raid-5 or Raid-6 sets) - combined with an
> appropriate file system (XFS comes out well here) you get massive
> scalability and very high parallel access speeds.
> 
> Other things to consider on big arrays are redundancy of controllers, or
> even servers (for SAN arrays).  Consider the pros and cons of spreading
> your redundancy across blocks.  For example, if your server has two
> controllers then you might want your low-level block to be Raid-1 pairs
> with one disk on each controller.  That could give you a better spread
> of bandwidths and give you resistance to a broken controller.
> 
> You could also talk about asymmetric raid setups, such as having a
> write-only redundant copy on a second server over a network, or as a
> cheap hard disk copy of your fast SSDs.

Snag is, I don't manage large arrays - it's a lot to think about. I
might add that later.
> 
> And you could also discuss strategies for disk replacement - after
> failures, or for growing the array.
> 
> It is also worth emphasising that RAID is /not/ a backup solution - that
> cannot be said often enough!
> 
> Discuss failure recovery - how to find and remove bad disks, how to deal
> with recovering disks from a different machine after the first one has
> died, etc.  Emphasise the importance of labelling disks in your machines
> and being sure you pull the right disk!
> 
I think that's covered elsewhere :-)

Cheers,
Wol