[CentOS] storage for mailserver

Thu Sep 17 20:25:56 UTC 2020
Phil Perry <pperry at elrepo.org>

On 17/09/2020 13:35, Michael Schumacher wrote:
> Hello Phil,
> Wednesday, September 16, 2020, 7:40:24 PM, you wrote:
> PP> You can achieve this with a hybrid RAID1 by mixing SSDs and HDDs, and
> PP> marking the HDD members as --write-mostly, meaning most of the reads
> PP> will come from the faster SSDs retaining much of the speed advantage,
> PP> but you have the redundancy of both SSDs and HDDs in the array.
> PP> Read performance is not far off native write performance of the SSD, and
> PP> writes mostly cached / happen in the background so are not so noticeable
> PP> on a mail server anyway.
> very interesting. Do you or anybody else have experience with this
> setup? Any test results to compare? I will do some testing if nobody
> can come up with comparisons.
> best regards
> ---
> Michael Schumacher

Here's a few performance stats from my setup, made with fio.

Firstly a RAID1 array from 2 x WD Black 1TB drives. Second set of 
figures are the same are for a RAID1 array with the same 2 WD Black 1TB 
drives and a WD Blue NVMe (PCIe X2) added into the array, with the 2 X 
HDDs set to --write-mostly.

Sequential write QD32
147MB/s (2 x HDD RAID1)
156MB/s (1 x NVMe, 2 x HDD RAID1)

The write tests give near identical performance with and without the SSD 
in the array as once any cache has been saturated, write speeds are 
presumably limited by the slowest device in the array.

Sequential read QD32
187MB/s (2 x HDD RAID1)
1725MB/s (1 x NVMe, 2 x HDD RAID1)

Sequential read QD1
162MB/s (2 x HDD RAID1)
1296MB/s (1 x NVMe, 2 x HDD RAID1)

4K random read
712kB/s (2 x HDD RAID1)
55.0MB/s (1 x NVMe, 2 x HDD RAID1)

The read speeds are a completely different story, and the array 
essentially performs identically to the native speed of the SSD device 
once the slower HDDs are set to --write-mostly, meaning the reads are 
prioritized to the SSD device. The SSD NVMe device is limited to PCIe X2 
hence why sequential read speeds top out at 1725MB/s. Current PCIe X4 
devices should be able to double that.

To summarize, a hybrid RAID1 mixing HDDs and SSDs will have write 
performance similar to the HDD (slowest device) and read performance 
similar to the SSD (fastest device) as long as the slower HDDs are added 
to the array with the --write-mostly flag set. Obviously these are 
synthetic I/O tests and may not reflect real world application 
performance but at least give you a good idea where the underlying 
bottlenecks may be.