I’m experiencing unusual behavior with my system. When I attempt to write to four disks concurrently. In fact, the last disk (/dev/sdd) consistently operates at a speed ranging from 40MB/s to 5MB/s. Recently, I purchased this item from Aliexpress https://www.aliexpress.com/item/1005005653706057.html You can see here a picture https://ibb.co/5jMd4p2
Is a M.2 to Sata3.0 Adapter Card 6 Port III 6GB/S Pci-E Controller Card equipped with the ASM1166 chip.
The system where it is installed is a MiniPC with an Intel N100 that has two M.2 slots. In one of these slots, there is an NVMe drive that contains the operating system (Ubuntu 24.04). The other slot is occupied by an adapter that controls four separate SATA HDDs, each with a capacity of 3TB. I prefer to use them individually rather than in RAID. Then i have 4 disks (/dev/sda /dev/sdb /dev/sdc /dev/sdd) mounted on 4 directories (/drv/diska /drv/diskb /drv/diskc /drv/diskd)
I conducted a system test using the command dd if=/dev/random of=/drv/disk[DiskLetter]/rand.txt bs=1M count=100. This command writes a random file to the [DiskLetter] disk. I executed this command simultaneously on all disks during the test.
Everything operates smoothly as I write to three disks simultaneously. In this setup, each disk maintains a writing speed of approximately 150MB/s. Consequently, I can achieve a combined writing speed of 450MB/s across the disks. I have the flexibility to choose any configuration I prefer. For instance, whether I write to sda, sdb, sdd, or sda, sdc, sdd, or even sda, sdb, sdc, the speed remains consistently around 450MB/s. Here you can see a screenshot of nmon when I write on three disks simultaneously https://ibb.co/h18NQJ2, here when I write only on one disk https://ibb.co/KFSG4bB
The issue arises when I attempt to write to four disks concurrently. In fact, the last disk (/dev/sdd) consistently operates at a speed ranging from 40MB/s to 5MB/s. The other disks perform well, maintaining a speed of about 150MB/s each. Here you can see a screenshot of nmon. https://ibb.co/Hhck7SS
I expected that the workload across the disks would be more distributed, with each disk achieving a speed of 100MB/s. However, it appears that the system prioritizes the first three disks in sequence, causing the fourth disk to suffer from insufficient bandwidth.
I’ve tried everything, including physically rearranging the disk order on the adapter and using different disks. However, the last disk (sdd) consistently shows a speed of 40MB/s to 5MB/s, while the others maintain a speed of around 150MB/s.
It appears that the I/O scheduler in the operating system is not balancing the data transmission effectively. It could also be an issue with the controller. I’m uncertain and would appreciate assistance in resolving this mystery.
