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.
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