On 3/23/19 11:51 AM, Rory Campbell-Lange wrote:
On 23/03/19, Andy Colson (andy@xxxxxxxxxxxxxxx) wrote:
On 3/23/19 7:09 AM, Rory Campbell-Lange wrote:
On 17/03/19, Rory Campbell-Lange (rory@xxxxxxxxxxxxxxxxxx) 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
