Re: software or hardware RAID?

On Sat, Mar 23, 2019 at 12:09:11PM +0000, Rory Campbell-Lange wrote:
> On 17/03/19, Rory Campbell-Lange (rory(at)campbell-lange(dot)net) wrote:
> > We aren't sure whether to use software MDRaid or a MegaRAID card.
> >
> > 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.
>
> Apologies for re-heating this email from last week. I could really do with the
> advice.
>
> 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
>
> Thanks for any advice.
> Rory

Hi Rory,

The main reason, in my opinion, to use a HW RAID card is for the NVRAM
battery backed cache to support writing to traditional spinning disks.
Since your SSDs have power-loss support, you do not need that and the HW
RAID controller. For database use, you would almost certainly be using
RAID 10 and software RAID 10 is extremely performant. I am in the middle
of setting up a new system with NVMe SSD drives and HW RAID would be a
terrible bottle-neck and software RAID is really the only realistice
option.

Regards,
Ken