[ClusterLabs] SAN with drbd and pacemaker

[Marco Marino]

ok, first if all, thank you for your answer. This is acomplicated task and
I cannot found many guides (if you have are welcome).
I'm using RAID6 and I have 20 disks of 4TB each.
In RAID6 space efficiency is 1-2/n, so a solution for small Virtual Drive
could be 4 or 5 disks. If I use 4 disks I will have (4*4) * (1-2/4) = 8 TB
of effective space. Instead, if I use 5 disks, I will have (5*4) * (1-2/5)
= 12TB of effective space.
Space efficiency is not a primary goal for me, I'm trying to reduce time of
rebuilding when a disk fails (and performance improvement!).

"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."

Can you explain me this? 16 volumes?

Thank you



2015-09-17 15:54 GMT+02:00 Kai Dupke <kdupke at suse.com>:

> 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
> --
> Sell not virtue to purchase wealth, nor liberty to purchase power.
> Phone:  +49-(0)5102-9310828     Mail: kdupke at suse.com
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