Re: SSD Drives

On Friday, April 4, 2014, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:

> On Fri, Apr 4, 2014 at 1:18 PM, John R Pierce <pierce(at)hogranch(dot)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 <mmoncure(at)gmail(dot)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