Re: software or hardware RAID?

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 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
>

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.

-Andy