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. Regards, Dennis _______________________________________________ drbd-user mailing list [email protected] http://lists.linbit.com/mailman/listinfo/drbd-user
