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

Adam Thompson athompso at athompso.net
Wed Sep 10 12:46:41 CDT 2014


On 14-09-10 03:51 AM, Trevor Cordes wrote:
> Final note: After hearing ZFS (on Linux at least) cannot grow by 1 disk
> when using RAID5 or 6, and after nearly 10 years using XFS on md on huge
> arrays, I say give some hero cookies to md/XFS.  It's withstood some
> pretty strange events on my server, and has never blown up.  If I'm wrong
> about the sunit being a problem, then md/XFS is a great option for those
> who want to gradually add space as they need it.

The fixed-topology limitation is endemic to ZFS' design, not just the 
Linux port.

However, there are many use cases where it's not a limitation: almost 
exactly the type of system we're trying to build.  If you only have 
(say) 8 drive bays, and you fill all 8 drive bays on day 1, you never 
need to grow the array by a single drive; the only two growth scenarios 
that are possible are dictated by your hardware:
     a) replace all 8 drives with larger drives - ZFS supports this
     b) add an external drive shelf with another (say) 8 drives - ZFS 
supports this, as a second sub-volume, effectively creating a RAID n+0 
(typically RAID60) volume.

ZFS has three major advantages for most people: 1) no RAID write-hole 
behaviour; 2) automatic resilvering; 3) integrated, effectively 
infinite, snapshots with built-in replication.

RHEL 7 (and thus CentOS 7, SL 7, etc.) defaults to XFS for the root file 
system.  Obviously you're not the only one who likes XFS!

I've generally found that the filesystem stripe width doesn't make a 
whole lot of difference on modern hardware; the worst-case I can recall 
encountering was actually due to block misalignment, not stripe width in 
the end.  I do recall it making a measurable difference on slow 5400rpm 
IDE drives with a controller that didn't do useful caching, about 10 
years ago.

-- 
-Adam Thompson
  athompso at athompso.net
  Cell: +1 204 291-7950
  Fax: +1 204 489-6515



More information about the Roundtable mailing list