On Friday, April 4, 2014, Scott Marlowe <scott.marl...@gmail.com> wrote:
> On Fri, Apr 4, 2014 at 1:18 PM, John R Pierce > <pie...@hogranch.com<javascript:;>> > wrote: > > On 4/4/2014 12:08 PM, Scott Marlowe wrote: > >> > >> You don't technically need the BBU / flashback memory IF the > >> controller is in write through. > > > > > > if you HAVE the BBU/flash why would you put the controller in write > > through?? the whole POINT of bbu/flashback is that you can safely enable > > writeback caching. > > > > my testing with postgresql OLTP benchmarks on Linux, I've found virtually > > identical performance using mdraid vs hardware raid in the same caching > > mode. its the writeback cache that gives raid cards like the LSI > Megaraid > > SAS2 series, or HP P420, or whatever, their big advantage vs a straight > JBOD > > configuration. > > I'm not sure you read / got the whole conversation. The OP was asking > if he COULD use a RAID controller with no BBU in write through with > SSDs. It's a valid question. My main point was in answer to this > response: > > On Fri, Apr 4, 2014 at 11:15 AM, Merlin Moncure > <mmonc...@gmail.com<javascript:;>> > wrote: > > On Fri, Apr 4, 2014 at 11:04 AM, Steve Crawford > >> 2. Do I need both BBU on the RAID *and* capacitor on the SSD or just on > one? > >> Which one? I'm suspecting capacitor on the SSD and write-through on the > >> RAID. > > > > You need both. The capacitor protects the drive, the BBU protects the > > raid controller. > > Context is king here. You do not have to have a BBU as long as you are > in write through as the OP mentioned. With no BBU, in write-through, > with supercaps, you should be safe. It's not a sensible configuration > for most applications. OTOH, most HW RAIDs have auto spare promotion > and easy swap out of dead drives with auto-rebuild. So if you're > building 1000 units for the government that just plug in and work, you > want the poor guy on the other end to just unplug bad drives and > replace them. The cost of a service call could be way more than a HW > RAID card. So, there are plenty of reasons you might want to test or even run > without a BBU. That wasn't my point. My point was you're SAFE (or > should be) with a HW RAID no BBU and supercapped SSDs. Agreed on all points. At the end of the day though hw raid is debatable in terms of value with ssd. I like mdadm more than most ulilitues also. merlin
