Resizing 'system_b' FAILED

[DanielR-I]

After another factory reset, my oem unlock option was finally enabled in my moto g play (2021).

Following the instructions here, I was making great headway:

$ fastboot reboot fastboot
Rebooting into fastboot OKAY [ 0.001s]
< waiting for any device >
Finished. Total time: 18.892s

$ fastboot erase system
Erasing 'system_b' OKAY [ 0.180s]
Finished. Total time: 0.225s

and then I got my first error trying to plash the uncompressed system-squeak-arm64-ab-floss.img.xz (AOSP 12.1 v416)

$ fastboot flash system system.img
Invalid sparse file format at header magic
Resizing 'system_b' FAILED (remote: 'Not enough space to resize partition')
fastboot: error: Command failed

Same result adding in the "force" paramater...

$ fastboot --force flash system system.img
Invalid sparse file format at header magic
Resizing 'system_b' FAILED (remote: 'Not enough space to resize partition')
fastboot: error: Command failed

Looking back at the instructions, I noticed the following:

Step #7: Reboot to fastbootd by running: "fastboot reboot fastboot" in CMD
Then run: fastboot erase system
Then: fastboot flash system system.img (Might get error message!) (To fix, run "fastboot delete-logical-partition product_a" Then run "fastboot flash system system.img" again)

I thought I would pause here and ask for advice as i find the "fix" odd... I know from my erase/flash command that _b is the context and system is the partition... so is it correct to now delete product_a... I don't want to make things any worse; perhaps there is a better approach?

[DanielR-I]

I think I found my answer by reading further in the instruction thread I linked... specifically where the poster shared their steps/results, proving that it was important to stick in the a/b context like I thought/questioned.

I'm now booted into the treble rom after the following:

$ fastboot delete-logical-partition product_b
Deleting 'product_b' OKAY [ 0.048s]
Finished. Total time: 0.048s

$ fastboot flash system system.img
Invalid sparse file format at header magic
Resizing 'system_b' OKAY [ 0.006s]
Sending sparse 'system_b' 1/10 (262096 KB) OKAY [ 6.563s]
Writing 'system_b' OKAY [ 4.413s]
Sending sparse 'system_b' 2/10 (262100 KB) OKAY [ 6.598s]
Writing 'system_b' OKAY [ 3.565s]
Sending sparse 'system_b' 3/10 (262112 KB) OKAY [ 6.602s]
Writing 'system_b' OKAY [ 3.535s]
Sending sparse 'system_b' 4/10 (262128 KB) OKAY [ 6.610s]
Writing 'system_b' OKAY [ 3.310s]
Sending sparse 'system_b' 5/10 (262124 KB) OKAY [ 6.641s]
Writing 'system_b' OKAY [ 3.576s]
Sending sparse 'system_b' 6/10 (262120 KB) OKAY [ 6.589s]
Writing 'system_b' OKAY [ 3.535s]
Sending sparse 'system_b' 7/10 (262120 KB) OKAY [ 6.600s]
Writing 'system_b' OKAY [ 3.246s]
Sending sparse 'system_b' 8/10 (262084 KB) OKAY [ 6.568s]
Writing 'system_b' OKAY [ 3.495s]
Sending sparse 'system_b' 9/10 (262140 KB) OKAY [ 6.583s]
Writing 'system_b' OKAY [ 3.423s]
Sending sparse 'system_b' 10/10 (44232 KB) OKAY [ 1.110s]
Writing 'system_b' OKAY [ 0.927s]
Finished. Total time: 93.965s

$ fastboot -w
Erasing 'userdata' OKAY [ 0.165s]
Erase successful, but not automatically formatting.
File system type raw not supported.
Erasing 'metadata' OKAY [ 0.006s]
Erase successful, but not automatically formatting.
File system type raw not supported.
Finished. Total time: 0.180s

$ fastboot reboot
Rebooting OKAY [ 0.000s]
Finished. Total time: 0.050s