[DRBD-user] Using DRBD as the Only RAID

Dennis Jacobfeuerborn dennisml at conversis.de
Thu Jul 24 04:57:25 CEST 2014

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On 24.07.2014 03:38, Robinson, Eric wrote:
> On 23.07.2014 23:42, Robinson, Eric wrote:
>>> Sounds like you want to run Raid1 (drbd) over Raid0 (ssd+md?). This 
>>> is more fragile than Raid0 over Raid1, so you might consider 
>>> implementing Raid0 using DM over multiple drbd devices, each mirroring a single ssd.
>>> Andreas
>> Our plan is to implement Raid0 using Intel hardware RAID controllers on each server, then Logical Raid1 would be provided by DRBD between the servers. My thought was that if anything happens to a RAID volume, DRBD would simply detach and start using the volume from the other server.  No?
> Yes, but there are two issues with this:
> a) Once you replaced the failed drive you have to re-sync the entire Raid-0.
> b) If a single drive fails on system b before you replaced and re-synced system a your data is gone.
> If on the other hand you build multiple DRBD mirrors with a Raid-0 on top:
> a) you only have to resync a single drive which is faster and doesn't impact the performance of the other mirrors.
> b) you only lose data if the other drive of the degraded mirror fails.
> If any other drive fails your data is still fine.
> So while the "Raid-0 over multiple DRBD mirrors" setup is a bit more finicky to handle it provides significantly higher reliability and faster rebuild times.
> ---
> That sounds like a very interesting solution, but I assume you are talking about using OS software RAID to build the Raid0 over multiple DRBD mirrors. In that case, don't you lose the benefit of the RAID controller's 1GB cache? And, more importantly, aren't you making yourself vulnerable to data corruption caused by power loss (because the cached data is not backed by the controller battery), kernel panic, or other OS issues?   

If you don't utilize the cache than you cannot lose the cache's contents
through a power outage or other failure so that isn't really a problem.
However you should be able to just pass the disks through individually
if the controller is decent. In that case you'd still benefit from the
battery backed cache.


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