Re: software or hardware RAID?

On 23/03/19, Andy Colson (andy(at)squeakycode(dot)net) wrote:
> On 3/23/19 7:09 AM, Rory Campbell-Lange wrote:
> > On 17/03/19, Rory Campbell-Lange (rory(at)campbell-lange(dot)net) wrote:
...
> > > We're buying some new Postgres servers with
> > >
> > > 2 x 240GB Intel SSD S4610 (RAID1 : system)
> > > 4 x 960GB Intel SSD S4610 (RAID10 : db)
> > >
> > > We'll be using Postgres 11 on Debian.
> > >
> > > The MegaRAID 9271-8i with flash cache protection is available from our
> > > provider. I think they may also have the 9361-8i which is 12Gb/s.
> > >
> > > Our current servers which use the LSI 9261 with SSDs and we don't see
> > > any IO significant load as we are in RAM most of the time and the RAID
> > > card seems to flatten out any IO spikes.
> > >
> > > We use MDRaid elsewhere but we've never used it for our databases
> > > before.

> > Has anyone got any general comments on whether software RAID or an LSI card
> > is preferable?
> >
> > We will be replicating load on an existing server, which has an LSI 9261 card.
> > Below is some stats from sar showing a "heavy" period of load on vdisk sda
> >
> > 00:00:01 DEV tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz await svctm %util
> > 14:15:01 sda 112.82 643.09 14986.24 138.53 2.09 18.50 0.25 2.86
> > 14:25:01 sda 108.52 270.17 15682.94 147.01 1.87 17.22 0.25 2.73
> > 14:35:01 sda 107.96 178.25 14868.52 139.37 1.70 15.73 0.23 2.53
> > 14:45:01 sda 150.97 748.94 16919.69 117.03 1.83 12.11 0.22 3.28

> I have run both software and hardware (though different than the card you listed), and had good success with both. In cases where I had little money, just drop 6 drives into a md raid 10, and run happy for years and years. I run production PG 11 on software raid 10 as we speek.
>
> I personally prefer software raid, for a few reasons:
> 1) you'll probably be running on a batter backup anyway, so missing raid card battery isn't that much
> 2) 100% compatible with any other hardware you wanna run. Sucky thing about hardware card is your on that one forever.
> 3) tooling is much better and simpler. I really hate the crappy bios raid screen. I never know if adding an HD to an exiting raid will wipe it or maintain it.
> 4) I setup smartctl to watch and report on drives. Even a 50% chance it detects before failure is a net benefit. You cant always to that through hardware raid
>
> You can always start with software raid, see how it runs for a while, then buy hardware raid if its not working out.

Thanks very much for the comments, Andy.

If money was no object, would you choose a fancy hardware RAID card?

You are right that the SSDs we are purchasing have enough cache + power to not need a BBU, and I agree that the management tools for software raid are much more convenient.

Rory