Tony Travis wrote: > Rahul Nabar wrote: >> If I have a option between doing Hardware RAID versus having software >> raid via mdadm is there a clear winner in terms of performance? Or is >> the answer only resolvable by actual testing? I have a fairly fast >> machine (Nehalem 2.26 GHz 8 cores) and 48 gigs of RAM. > > Hello, Rahul. > > It depends which level of RAID you want to use, and if you want hot-swap > capability. I use inexpensive 3ware 8006-2 RAID1 controllers and stripe > them using "md" software RAID0 to make RAID10 arrays. This gives me good > performance and hot-swap capability (the production md RAID driver does > not support hot-swap). However, where "md" really scores is portability. > My RAID's can only be read by 3ware controllers - I made a considered > descision about this: The 3ware controllers are well-supported by Linux > kernels, but it makes me uneasy using a proprietary RAID format. I do > also use "md" RAID5 which is more space efficient, but read this: > > http://www.baarf.com/
Hot swap is at least partially dependent on the controller. Even most of the built-in controllers now support hot swap. I'm not aware of md hardcoding anything on boot which would prevent a change on the fly, except that you would manually have to initiate a rebuild after swapping. Could you be more specific about what wasn't working as far as hot swapping? Was this with current controllers? Which ones? >> Should I be using the vendor's hardware RAID or mdadm? In case a >> generic answer is not possible, what might be a good way to test the >> two options? Any other implications that I should be thinking about? > > In fact, "mdadm" is just the user-space command for controlling the "md" > driver. The problem with using an on-board RAID controller is that many > of these are 'host' RAID (i.e. need a Windows driver to do the RAID) in > which case you are using the CPU anyway, and they also use proprietary > formats. Generally, I just use SATA mode on the on-board RAID controller > and create an "md" RAID. This means that I can replace a motherboard > withour worrying if it has the same type of RAID controller on-board. Yes, it's pedigreed software instead of mystery meat firmware. >> Finally, there;s always hybrid approaches. I could have several small >> RAID5's at the hardware level (RIAD5 seems ok since I have smaller >> disks ~300 GB so not really in the domain where the RAID6 arguments >> kick in, I think) Then using LVM I can integrate storage while asking >> LVM to stripe across these RAID5's. Thus I'd get striping at two >> levels: LVM (software) and RAID5 (hardware). > > Yes, I think a hybrid approach is good because that's what I use ;-) > > However, I would avoid relying on LVM mirroring for data protection. It > is much safer to stripe a set of RAID1's using LVM. I don't think LVM is > useful unless you are managing a disk farm. The commonest issue in disk > perfomance is decoupling seeks between different spindles, so I put the > system files on a different RAID1-set to /export (or /home) filesystems. LVM is there as a management convenience. It allows you to grow your disk pool more-or-less on demand. Where it's really beautiful, though, is when you want to migrate data -- Geoffrey D. Jacobs _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
