[ClusterLabs] SAN with drbd and pacemaker

[Kai Dupke]

On 09/17/2015 09:44 AM, Marco Marino wrote:
> Hi, I have 2 servers supermicro lsi 2108 with many disks (80TB) and I'm
> trying to build a SAN with drbd and pacemaker. I'm studying, but I have no
> experience on large array of disks with drbd and pacemaker, so I have some
> questions:
> 
> I'm using MegaRAID Storage Manager to create virtual drives. Each virtual
> drive is a device on linux (eg /dev/sdb, /dev/sdc.....), so my first
> question is: it's a good idea to create virtual drive of 8 TB (max)? I'm
> thinking to rebuild array time in case of disk failure (about 1 day for 8

It depends on your disks and RAID level. If one disk fails the content
of this disk has to be recreated by either copying (all RAID levels with
some RAID 1 included) or calculating (all with no RAID1 included), in
the later case all disks get really stressed.

If you run 20x4TB disks as RAID6, then an 8TB volume is only ~500G per
disk. However, if one disk fails, then all the other 15 volumes this
disk handles are broken, too. (BTW, most raid controller can handle
multiple stripes per disk, but usually only a handful) In such case the
complete 4TB of the broken disk has to be recovered, affecting all 16
volumes.

On the other side, if you use 4x5x4TB as 4x 12TB RAID6, a broken disk
only affects one of 4 volumes - but at the cost of more disks needed.

You can do the similar calculation based on RAID16/15.

The only reason I see to create small slices is to make them fit on
smaller replacement disks, which might be more easily available/payable
at time of error (but now we are entering a more low cost area where
usually SAN and DRBD do not take place).

greetings
Kai Dupke
Senior Product Manager
Server Product Line
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Phone:  +49-(0)5102-9310828     Mail: kdupke at suse.com
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