[PLUG] RAID6 or RAID5+HS?

[JP Vossen <jp@jpsdomain.org>]

Semi OT but my argument is that the OS going on the hardware is 
Debian...  :-)  Note, Debian Jessie requires 
"firmware-bnx2_0.43_all.deb" for the NICs on the R710.  I haven't tested 
the R720 yet.

I want to rebuild my VM server and the hardware I'll probably use is a 
Dell PE R710 with 6x3T drives and a PERC H700 (2.02-0025) [1].  I can do 
RAID6 or RAID5+hot-spare, and either way I should get about 11T out of 
it.  All are 7200RPM SATA disks.

As I understand it, either RAID6 or RAID5+HS can handle losing 2 disks, but:
* RAID6 has a write penalty for the extra parity block
* Hot-spare has a gap while the array rebuilds onto the hot-spare

The use-case is my main VMware server (Debian Jessie + LXDE + VMware 
Workstation 10.x [2].  The critical VM is my main "services" server 
(DNS, DHCP, file&print, etc., also Jessie) and the next important one 
will be my MythTV backend (Mythbuntu 14.04), once I virtualize it [3]. 
Everything else is just test VMs and whatever.  I do not backup the 
contents of the MythTV server because it's too big, but everything else 
is backed up in a few different ways (BackupPC, BoxBackup, rsync).  I've 
got 1.5T in VMs and 3T in MythTV, so 11T is lots of room.

I don't do a lost of disk-intensive things.  Probably MythTV recording 2 
shows at once while playing a third is the worst, but I really don't 
have a good sense of the relative load and capacities of all the parts 
involved.

This will be most of my eggs in one basket, so I'd really like tolerance 
for 2 disks to fail close together.  So...RAID6 or RAID5+HS?  Any other 
random thoughts?

Thanks,
JP
_____________________________________________________
Footnotes:

[1] I might also be able to use an R720 with 8x4T drives, but that has 1 
bad drive already and I may end up using that for a $WORK thing.  It was 
drawing 150-180 watts during a RAID init, but I'd like to re-test with 
an OS with CPU scaling installed and running.


[2] I know that in theory I could use ESXi, but:
1) I have 100+ VMs with many snapshots, and I don't know of any way to 
move those VMs from Workstation into ESXi without:
	1.1) doing it manually and
	1.2) losing snapshots (show stopper)
2) The last time I tried using ESXi (5.x IIRC) I found it impossible to 
do anything useful in it
3) I do not have, nor will I have, any Windows involved in any VM 
control or management, or anything important

Those last two may have changed since newer ESXi has the web GUI and not 
the fat Windows client.  But the show-stopping snapshot issue remains.


[3] MythTV is currently on a PE2950 with 6x1T drives in RAID5, but the 
unit is drawing about 300 watts.  My current R710 with 6x1T RAID5 is 
drawing about 140 watts, so I really wish I'd bothered to put the 
kill-a-watt on them sooner.  All have dual power supplies so the test is 
easy (and yes, I unplugged the non-kill-a-watt supply for the test).

--  -------------------------------------------------------------------
JP Vossen, CISSP | http://www.jpsdomain.org/ | http://bashcookbook.com/
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