On 19/09/2020 19:19, Chris Schanzle via CentOS wrote:
On 9/17/20 4:25 PM, Phil Perry wrote:
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.
Too bad the 4k random write tests are missing above.
4k random writes QD1
with fsync=1
56.6kB/s (2 x HDD RAID1)
77.8kB/s (1 x NVMe, 2 x HDD RAID1)
with fsync=1000
1431kB/s (2 x HDD RAID1)
1760kB/s (1 x NVMe, 2 x HDD RAID1)
I have used SSD + HDD RAID1 configurations in dozens of CentOS desktops and servers for years and it works very well with the --write-mostly flag being set on the HDD. With most reads coming from the SSD, starting programs are much quicker.
However, I find the write queue to be very, very small, so the system "feels" like a slow HDD system during writing.
Yes, as per above, 4k random write performance is similar to that of a
pure HDD RAID array.
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