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

Wols Lists
2018-01-29T16:38:43Z

On 29/01/18 15:23, Johannes Thumshirn wrote:
> Hi linux-raid, lsf-pc
>
> (If you've received this mail multiple times, I'm sorry, I'm having
> trouble with the mail setup).

My immediate reactions as a lay person (I edit the raid wiki) ...
>
> With the rise of bigger and bigger disks, array rebuilding times start
> skyrocketing.

And? Yes, your data is at risk during a rebuild, but md-raid throttles
the i/o, so it doesn't hammer the system.
>
> In a paper form '92 Holland and Gibson [1] suggest a mapping algorithm
> similar to RAID5 but instead of utilizing all disks in an array for
> every I/O operation, but implement a per-I/O mapping function to only
> use a subset of the available disks.
>
> This has at least two advantages:
> 1) If one disk has to be replaced, it's not needed to read the data from
> all disks to recover the one failed disk so non-affected disks can be
> used for real user I/O and not just recovery and

Again, that's throttling, so that's not a problem ...

> 2) an efficient mapping function can improve parallel I/O submission, as
> two different I/Os are not necessarily going to the same disks in the
> array.
>
> For the mapping function used a hashing algorithm like Ceph's CRUSH [2]
> would be ideal, as it provides a pseudo random but deterministic mapping
> for the I/O onto the drives.
>
> This whole declustering of cause only makes sense for more than (at
> least) 4 drives but we do have customers with several orders of
> magnitude more drivers in an MD array.

If you have four drives or more - especially if they are multi-terabyte
drives - you should NOT be using raid-5 ...
>
> At LSF I'd like to discuss if:
> 1) The wider MD audience is interested in de-clusterd RAID with MD

I haven't read the papers, so no comment, sorry.

> 2) de-clustered RAID should be implemented as a sublevel of RAID5 or
> as a new personality

Neither! If you're going to do it, it should be raid-6.

> 3) CRUSH is a suitible algorith for this (there's evidence in [3] that
> the NetApp E-Series Arrays do use CRUSH for parity declustering)
>
> [1] http://www.pdl.cmu.edu/PDL-FTP/Declustering/ASPLOS.pdf
> [2] https://ceph.com/wp-content/uploads/2016/08/weil-crush-sc06.pdf
> [3]
> https://www.snia.org/sites/default/files/files2/files2/SDC2013/presentations/DistributedStorage/Jibbe-Gwaltney_Method-to_Establish_High_Availability.pdf
>
Okay - I've now skimmed the crush paper [2]. Looks well interesting.
BUT. It feels more like btrfs than it does like raid.

Btrfs manages disks, and does raid, it tries to be the "everything
between the hard drive and the file". This crush thingy reads to me like
it wants to be the same. There's nothing wrong with that, but md is a
unix-y "do one thing (raid) and do it well".

My knee-jerk reaction is if you want to go for it, it sounds like a good
idea. It just doesn't really feel a good fit for md.

Cheers,
Wol