Re: software or hardware RAID?

On 3/23/19 11:51 AM, Rory Campbell-Lange wrote:
> 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
>

That's a tough question. I don't even have SSD's yet. I can't say what performance is like on HW Raid SSD vs SW Raid SSD. If money where no object, and I wanted performance over all else, I'd benchmark both.

If usability were >= performance, I'd go with extra ram, and SW Raid SSD. And a BMW. :-)

-Andy