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.