[RndTbl] linux md RAID6 + XFS + add 1 drive

[Adam Thompson]

On 2014-09-10 16:59, Trevor Cordes wrote:
> Outside of the enterprise space, I can see lots of scenarios where you
> want to add a less-than-double amount of disks to an existing array.
> Especially for home use, where you only want to do the big $1500 outlay
> on disks every 3-6 years (generally when capacities per $ have
> doubled).

Yup.  Although even in the home-user situation, there are a lot of 4-bay 
enclosures where adding one more disk can't happen anyway.

> If you find yourself in the "shoot, we should have bought 1 more disk"
> after your array is full, then having no option but buying another X
> drives when your budget is spent kind of sucks.  The ability to solve a
> full-array problem by spending just $150 on one disk at any time is very
> attractive.

I definitely agree with you on that point.

ZFS has three critical flaws from my perspective:
1) needs way too much RAM for deduplication
2) license (although FreeBSD and Linux both work around this quite well)
3) inability to alter raid volume topology after creation

> I have a funny feeling that md and the FS's on top of it will slowly
> add many ZFS features to be almost as rich as ZFS.

Well... yes and no.  The development is mostly happening on btrfs, which 
is intended to be a head-to-head competitor to ZFS, including the 
ability to integrate the md layer into the filesystem layer directly. 
(This allows for much more intelligent - i.e. faster - failed-drive 
rebuilds, if the array isn't close to being full.)

DragonFlyBSD has also done some amazing work on the HammerFS filesystem, 
which was designed as a "better-than-ZFS" option from the outset.  It's 
not yet ported to any other OS (AFAIK) and lacks true built-in RAID 
functions, but in other ways is a very exciting filesystem.

I guess you could say that ZFS is already a "legacy" filesystem, in the 
sense that it's well-established, widely-adopted, and it already has 
competitors nipping at its heels.  XFS is in pretty much the same boat, 
but is a little older, and relies on a RAID block-layer device like most 
filesystems.  Not to say these aren't valid choices today - they most 
certainly are!

Most of the coming-just-around-the-corner filesystem work appears to be 
happening on btrfs and Hammer... and based on personal experiences, I 
wouldn't want to run btrfs in production yet, and Hammer apparently also 
still has some quirks for the unwary.

-- 
-Adam Thompson
  athompso at athompso.net