On Wed, Jul 19, 2023 at 12:51:39PM +0200, Kai Tomerius wrote: > > In answer to Kai's original question, the setup that was described > > should be fine --- assuming high quality hardware. > > I wonder how to judge that ... it's an eMMC supposedly complying to > some JEDEC standard, so it *should* be ok. JEDEC promulgates the eMMC interface specification. That's the interface used to talk to the device, much like SATA and SCSI and NVMe. The JEDEC eMMC specification says nothing about the quality of the implementation of the FTL, or whether it is safe from power drops, or how many wirte cycles are supported before the eMMC soldered on the $2000 MCU would expire. If you're a cell phone manufacturer, the way you judge it is *before* you buy a few million of the eMMC devices, you subject the samples to a huge amount of power drops and other torture tests (including verifying the claimed number of write cycles in spec sheet), before the device is qualified for use in your product. > But on another aspect: how about the interaction between dm-integrity > and ext4? Sure, they each have their own journal, and they're > independent layers. Is there anything that could go wrong, say a block > that can't be recovered in the dm-integrity layer, causing ext4 to run > into trouble, e.g., an I/O error that prevents ext4 from mounting? > > I assume tne answer is "No", but can I be sure? If there are I/O errors, with or without dm-integrity, you can have problems. dm-integrity will turn bit-flips into hard I/O errors, but a bit-flip might cause silent file system cocrruption (at least at first), such that when you finally notice that there's a problem, several days or weeks or months may have passed, the data loss might be far worse. So turning an innocous bit flip into a hard I/O error can be a feature, assuming that you've allowed for it in your system architecture. If you assume that the hardware doesn't introduce I/O errors or bit flips, and if you assume you don't have any attackers trying to corrupt the block device with bit flips, then sure, nothing will go wrong. You can buy perfect hardware from the same supply store where high school physics teachers buy frictionless pulleys and massless ropes. :-) Cheers, - Ted
