[CentOS] storage for mailserver

Wed Sep 16 17:40:24 UTC 2020
Phil Perry <pperry at elrepo.org>

On 16/09/2020 17:11, Michael Schumacher wrote:
> hi,
> I am planning to replace my old CentOS 6 mail server soon. Most details
> are quite obvious and do not need to be changed, but the old system
> was running on spinning discs and this is certainly not the best
> option for todays mail servers.
> With spinning discs, HW-RAID6 was the way to go to increase reliability
> and speed.
> Today, I get the feeling, that traditional RAID is not the best
> option for SSDs. I am reading that all RAID members in SSD-arrays age
> synchronously so that the risk of a massive failure of more than one
> disk is more likely than with HDDs. There are many other concerns like
> excessive write load compared to non-raid systems, etc.
> Is there any common sense what disk layout should be used these days?
> I have been looking for some kind of master-slave system, where the
> (one or many) SSD is taking all writes and reads, but the slave HDD
> runs in parallel as a backup system like in a RAID1 system. Is there
> any such system?
> Any thoughts?

You can achieve this with a hybrid RAID1 by mixing SSDs and HDDs, and 
marking the HDD members as --write-mostly, meaning most of the reads 
will come from the faster SSDs retaining much of the speed advantage, 
but you have the redundancy of both SSDs and HDDs in the array.

Read performance is not far off native write performance of the SSD, and 
writes mostly cached / happen in the background so are not so noticeable 
on a mail server anyway.

I kind of stumbled across this setup by accident when I added an NVMe 
SSD to an existing RIAD1 array consisting of 2 HDDs.

# cat /proc/mdstat
Personalities : [raid1]
md127 : active raid1 sda1[2](W) sdb1[4](W) nvme0n1p1[3]
       485495616 blocks super 1.0 [3/3] [UUU]
       bitmap: 3/4 pages [12KB], 65536KB chunk

See how we have 3 devices in the above RAID1 array, 2 x HDDs, marked 
with a (W) indicating they are in --write-mostly mode, and one SSD 
(MVMe) device. I just went for 3 devices in the array as it started life 
as a 2 x HDD array and I added the third SSD device, but you can mix and 
match to suit your needs.

See the following article which may be helpful or search 'mdadm 
write-mostly' for more info.