Thank you very much for your answer.
I think you are right with your assumtion about the md superblock version 0.9.
I will add the kernel log at the end of this message.
Do you have a proposal about the resizing of the root and boot partition and filesystem sizes? I will follow additional proposals to bring this feature hopefully up and running.
I am an experienced Linux user, but currently i have to learn some things about the u-boot loader, OpenWRT early boot process and probably OpenWRT building skills.
[ 0.000000] printk: bootconsole [udbg0] enabled
[ 0.000000] Linux version 5.4.154 (builder@buildhost) (gcc version 8.4.0 (OpenWrt GCC 8.4.0 r16325-88151b8303)) #0 Sun Oct 24 09:01:35 2021
[ 0.000000] Using PowerPC 44x Platform machine description
[ 0.000000] -----------------------------------------------------
[ 0.000000] phys_mem_size = 0x10000000
[ 0.000000] dcache_bsize = 0x20
[ 0.000000] icache_bsize = 0x20
[ 0.000000] cpu_features = 0x0000000000000120
[ 0.000000] possible = 0x0000000040000120
[ 0.000000] always = 0x0000000000000120
[ 0.000000] cpu_user_features = 0x8c008000 0x00000000
[ 0.000000] mmu_features = 0x00000008
[ 0.000000] -----------------------------------------------------
[ 0.000000] Top of RAM: 0x10000000, Total RAM: 0x10000000
[ 0.000000] Memory hole size: 0MB
[ 0.000000] Zone ranges:
[ 0.000000] Normal [mem 0x0000000000000000-0x000000000fffffff]
[ 0.000000] Movable zone start for each node
[ 0.000000] Early memory node ranges
[ 0.000000] node 0: [mem 0x0000000000000000-0x000000000fffffff]
[ 0.000000] Initmem setup node 0 [mem 0x0000000000000000-0x000000000fffffff]
[ 0.000000] On node 0 totalpages: 65536
[ 0.000000] Normal zone: 576 pages used for memmap
[ 0.000000] Normal zone: 0 pages reserved
[ 0.000000] Normal zone: 65536 pages, LIFO batch:15
[ 0.000000] MMU: Allocated 1088 bytes of context maps for 255 contexts
[ 0.000000] pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
[ 0.000000] pcpu-alloc: [0] 0
[ 0.000000] Built 1 zonelists, mobility grouping on. Total pages: 64960
[ 0.000000] Kernel command line: root=/dev/sda2 rw rootfstype=squashfs,ext4 console=ttyS0,115200
[ 0.000000] Dentry cache hash table entries: 32768 (order: 5, 131072 bytes, linear)
[ 0.000000] Inode-cache hash table entries: 16384 (order: 4, 65536 bytes, linear)
[ 0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
[ 0.000000] Memory: 251696K/262144K available (6436K kernel code, 236K rwdata, 744K rodata, 180K init, 229K bss, 10448K reserved, 0K cma-reserved)
[ 0.000000] Kernel virtual memory layout:
[ 0.000000] * 0xfffdf000..0xfffff000 : fixmap
[ 0.000000] * 0xd1000000..0xfffdf000 : vmalloc & ioremap
[ 0.000000] SLUB: HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
[ 0.000000] NR_IRQS: 512, nr_irqs: 512, preallocated irqs: 16
[ 0.000000] UIC0 (32 IRQ sources) at DCR 0xc0
[ 0.000000] UIC1 (32 IRQ sources) at DCR 0xd0
[ 0.000000] UIC2 (32 IRQ sources) at DCR 0xe0
[ 0.000000] UIC3 (32 IRQ sources) at DCR 0xf0
[ 0.000000] random: get_random_u32 called from 0xc07059e4 with crng_init=0
[ 0.000000] time_init: decrementer frequency = 800.000008 MHz
[ 0.000000] time_init: processor frequency = 800.000008 MHz
[ 0.000020] clocksource: timebase: mask: 0xffffffffffffffff max_cycles: 0xb881274fa3, max_idle_ns: 440795210636 ns
[ 0.010286] clocksource: timebase mult[1400000] shift[24] registered
[ 0.016603] clockevent: decrementer mult[ccccccef] shift[32] cpu[0]
[ 0.022897] pid_max: default: 32768 minimum: 301
[ 0.027562] Mount-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
[ 0.034803] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
[ 0.046859] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645041785100000 ns
[ 0.056534] futex hash table entries: 256 (order: -1, 3072 bytes, linear)
[ 0.065967] NET: Registered protocol family 16
[ 0.070921] DMA: preallocated 256 KiB pool for atomic allocations
[ 0.078989] 256k L2-cache enabled
[ 0.082330] PCIE0: Port disabled via device-tree
[ 0.087528] PCI: Probing PCI hardware
[ 0.105313] SCSI subsystem initialized
[ 0.109752] libata version 3.00 loaded.
[ 0.114693] workqueue: max_active 576 requested for napi_workq is out of range, clamping between 1 and 512
[ 0.125469] clocksource: Switched to clocksource timebase
[ 0.131515] thermal_sys: Registered thermal governor 'step_wise'
[ 0.131730] NET: Registered protocol family 2
[ 0.142057] IP idents hash table entries: 4096 (order: 3, 32768 bytes, linear)
[ 0.149710] tcp_listen_portaddr_hash hash table entries: 512 (order: 0, 4096 bytes, linear)
[ 0.157984] TCP established hash table entries: 2048 (order: 1, 8192 bytes, linear)
[ 0.165580] TCP bind hash table entries: 2048 (order: 1, 8192 bytes, linear)
[ 0.172568] TCP: Hash tables configured (established 2048 bind 2048)
[ 0.178944] UDP hash table entries: 256 (order: 0, 4096 bytes, linear)
[ 0.185398] UDP-Lite hash table entries: 256 (order: 0, 4096 bytes, linear)
[ 0.192434] NET: Registered protocol family 1
[ 0.196742] PCI: CLS 0 bytes, default 32
[ 0.204422] dw_dmac 4bffd0800.dma: DesignWare DMA Controller, 2 channels
[ 0.216038] workingset: timestamp_bits=14 max_order=16 bucket_order=2
[ 0.227781] squashfs: version 4.0 (2009/01/31) Phillip Lougher
[ 0.302555] Block layer SCSI generic (bsg) driver version 0.4 loaded (major 252)
[ 0.310774] GPIO line 473 (Enable Reset Button, disable NOR) hogged as output/low
[ 0.318985] Serial: 8250/16550 driver, 16 ports, IRQ sharing enabled
[ 0.327421] printk: console [ttyS0] disabled
[ 0.331679] 4ef600300.serial: ttyS0 at MMIO 0x4ef600300 (irq = 33, base_baud = 462962) is a TI16750
[ 0.340645] printk: console [ttyS0] enabled
[ 0.348917] printk: bootconsole [udbg0] disabled
[ 0.362155] loop: module loaded
[ 0.365291] Loading iSCSI transport class v2.0-870.
[ 0.376487] sata-dwc 4bffd1000.sata: id 0, controller version 1.91
[ 0.383889] scsi host0: sata-dwc
[ 0.387465] ata1: SATA max UDMA/133 irq 40
[ 0.391715] sata-dwc 4bffd1800.sata: id 0, controller version 1.91
[ 0.399210] scsi host1: sata-dwc
[ 0.402688] ata2: SATA max UDMA/133 irq 41
[ 0.407565] libphy: Fixed MDIO Bus: probed
[ 0.411673] PPC 4xx OCP EMAC driver, version 3.54
[ 0.416789] MAL v2 /plb/mcmal, 1 TX channels, 1 RX channels
[ 0.422565] RGMII /plb/opb/emac-rgmii@ef601500 initialized with MDIO support
[ 0.429765] TAH /plb/opb/emac-tah@ef601350 initialized
[ 0.435210] /plb/opb/emac-rgmii@ef601500: input 0 in rgmii mode
[ 0.441267] libphy: emac_mdio: probed
[ 0.449628] eth0: EMAC-0 /plb/opb/ethernet@ef600c00, MAC 00:90:a9:be:fc:b1
[ 0.456503] eth0: found Broadcom BCM50610 PHY (0x01)
[ 0.461551] i2c /dev entries driver
[ 0.465118] booke_wdt: powerpc book-e watchdog driver loaded
[ 0.485767] NET: Registered protocol family 10
[ 0.491948] Segment Routing with IPv6
[ 0.495712] NET: Registered protocol family 17
[ 0.500198] bridge: filtering via arp/ip/ip6tables is no longer available by default. Update your scripts to load br_netfilter if you need this.
[ 0.513112] 8021q: 802.1Q VLAN Support v1.8
[ 0.517356] drmem: No dynamic reconfiguration memory found
[ 0.523314] usb-regulator GPIO handle specifies active low - ignored
[ 0.755680] ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[ 0.761955] ata2: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[ 0.769168] ata1.00: ATA-9: WDC WD30EFRX-68AX9N0, 80.00A80, max UDMA/133
[ 0.775868] ata1.00: 5860533168 sectors, multi 0: LBA48 NCQ (depth 1/32)
[ 0.785137] ata1.00: configured for UDMA/133
[ 0.789813] scsi 0:0:0:0: Direct-Access ATA WDC WD30EFRX-68A 0A80 PQ: 0 ANSI: 5
[ 0.799069] sd 0:0:0:0: [sda] 5860533168 512-byte logical blocks: (3.00 TB/2.73 TiB)
[ 0.806816] sd 0:0:0:0: [sda] 4096-byte physical blocks
[ 0.812570] sd 0:0:0:0: [sda] Write Protect is off
[ 0.817369] sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
[ 0.822531] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[ 0.844975] ata2.00: ATA-9: WDC WD30EFRX-68AX9N0, 80.00A80, max UDMA/133
[ 0.851742] ata2.00: 5860533168 sectors, multi 0: LBA48 NCQ (depth 1/32)
[ 0.862025] ata2.00: configured for UDMA/133
[ 0.866671] scsi 1:0:0:0: Direct-Access ATA WDC WD30EFRX-68A 0A80 PQ: 0 ANSI: 5
[ 0.875941] sd 1:0:0:0: [sdb] 5860533168 512-byte logical blocks: (3.00 TB/2.73 TiB)
[ 0.883687] sd 1:0:0:0: [sdb] 4096-byte physical blocks
[ 0.889316] sd 1:0:0:0: [sdb] Write Protect is off
[ 0.894124] sd 1:0:0:0: [sdb] Mode Sense: 00 3a 00 00
[ 0.899478] sd 1:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[ 0.909538] sda: sda1 sda2 sda3
[ 0.915017] sd 0:0:0:0: [sda] Attached SCSI disk
[ 0.973490] sdb: sdb1 sdb2 sdb3
[ 0.978058] sd 1:0:0:0: [sdb] Attached SCSI disk
[ 0.982817] md: Waiting for all devices to be available before autodetect
[ 0.989598] md: If you don't use raid, use raid=noautodetect
[ 0.995726] md: Autodetecting RAID arrays.
[ 1.034796] md: invalid raid superblock magic on sda1
[ 1.039850] md: sda1 does not have a valid v0.90 superblock, not importing!
[ 1.058411] md: invalid raid superblock magic on sda2
[ 1.063461] md: sda2 does not have a valid v0.90 superblock, not importing!
[ 1.070555] md: invalid raid superblock magic on sda3
[ 1.075600] md: sda3 does not have a valid v0.90 superblock, not importing!
[ 1.120423] md: invalid raid superblock magic on sdb1
[ 1.125471] md: sdb1 does not have a valid v0.90 superblock, not importing!
[ 1.144642] md: invalid raid superblock magic on sdb2
[ 1.149688] md: sdb2 does not have a valid v0.90 superblock, not importing!
[ 1.156786] md: invalid raid superblock magic on sdb3
[ 1.161837] md: sdb3 does not have a valid v0.90 superblock, not importing!
[ 1.168780] md: autorun ...
[ 1.171574] md: ... autorun DONE.
[ 1.185138] EXT4-fs (sda2): warning: mounting unchecked fs, running e2fsck is recommended
[ 1.205693] EXT4-fs (sda2): mounted filesystem without journal. Opts: (null)
[ 1.212795] VFS: Mounted root (ext4 filesystem) on device 8:2.
[ 1.219382] Freeing unused kernel memory: 180K
[ 1.223893] Run /sbin/init as init process
[ 1.324861] init: Console is alive
[ 1.328496] init: - watchdog -
[ 1.343780] random: fast init done
[ 1.370270] kmodloader: loading kernel modules from /etc/modules-boot.d/*
[ 1.407406] usbcore: registered new interface driver usbfs
[ 1.412982] usbcore: registered new interface driver hub
[ 1.418351] usbcore: registered new device driver usb
[ 1.441240] dwc2 4bff80000.usbotg: 4bff80000.usbotg supply vusb_d not found, using dummy regulator
[ 1.450311] dwc2 4bff80000.usbotg: 4bff80000.usbotg supply vusb_a not found, using dummy regulator
[ 1.561835] dwc2 4bff80000.usbotg: DWC OTG Controller
[ 1.566923] dwc2 4bff80000.usbotg: new USB bus registered, assigned bus number 1
[ 1.574346] dwc2 4bff80000.usbotg: irq 36, io mem 0x4bff80000
[ 1.580936] hub 1-0:1.0: USB hub found
[ 1.584901] hub 1-0:1.0: 1 port detected
[ 1.595942] usbcore: registered new interface driver usb-storage
[ 1.602544] kmodloader: done loading kernel modules from /etc/modules-boot.d/*
[ 1.612461] init: - preinit -
[ 1.774053] eth0: link is down
[ 1.786911] random: procd: uninitialized urandom read (4 bytes read)
[ 4.221732] eth0: link is up, 1000 FDX, pause enabled
[ 4.226834] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[ 5.872332] mount_root: mounting /dev/root
[ 5.894212] EXT4-fs (sda2): re-mounted. Opts: (null)
[ 5.899398] mount_root: loading kmods from internal overlay
[ 5.915145] kmodloader: loading kernel modules from //etc/modules-boot.d/*
[ 5.923023] kmodloader: done loading kernel modules from //etc/modules-boot.d/*
[ 6.776216] block: attempting to load /etc/config/fstab
[ 6.794010] block: extroot: not configured
[ 6.821381] EXT4-fs (sda1): warning: mounting unchecked fs, running e2fsck is recommended
[ 6.850534] EXT4-fs (sda1): mounted filesystem without journal. Opts: (null)
[ 6.859244] urandom-seed: Seeding with /etc/urandom.seed
[ 6.898244] procd: - early -
[ 6.901194] procd: - watchdog -
[ 7.085346] random: crng init done
[ 7.455591] procd: - watchdog -
[ 7.466156] procd: - ubus -
[ 7.520792] procd: - init -
[ 8.136204] kmodloader: loading kernel modules from /etc/modules.d/*
[ 8.217866] device-mapper: ioctl: 4.41.0-ioctl (2019-09-16) initialised: dm-devel@redhat.com
[ 8.327746] xt_time: kernel timezone is -0000
[ 8.393634] kmodloader: done loading kernel modules from /etc/modules.d/*
[ 8.461393] urngd: v1.0.2 started.
[ 10.301970] md: md127 stopped.
[ 10.317619] md: kicking non-fresh sdb3 from array!
[ 10.341572] md/raid1:md127: active with 1 out of 2 mirrors
[ 10.358754] md127: detected capacity change from 0 to 2999383949312
[ 12.113911] EXT4-fs (md127): mounted filesystem with ordered data mode. Opts:
[ 12.397134] eth0: link is up, 1000 FDX, pause enabled
[ 12.414938] br-lan: port 1(eth0) entered blocking state
[ 12.420201] br-lan: port 1(eth0) entered disabled state
[ 12.425673] device eth0 entered promiscuous mode
[ 13.397546] br-lan: port 1(eth0) entered blocking state
[ 13.402787] br-lan: port 1(eth0) entered forwarding state
[ 13.412204] IPv6: ADDRCONF(NETDEV_CHANGE): br-lan: link becomes ready