usb/msc: erase blocks before writing to them #5017
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aykevl
commented
Aug 27, 2025
src/machine/usb/msc/msc.go
Outdated
| @@ -73,7 +73,7 @@ func newMSC(dev machine.BlockDevice) *msc { | |||
| m := &msc{ | |||
| // Some platforms require reads/writes to be aligned to the full underlying hardware block | |||
| blockCache: make([]byte, dev.WriteBlockSize()), | |||
| blockSizeUSB: 512, | |||
| blockSizeUSB: max(uint32(dev.EraseBlockSize()), 512), | |||
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Any idea why this was 512? If the block size is lower (128 for example), could this use a lower block size instead to reduce wear?
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The short version is that 512 bytes has been a near-universal standard sector size for disk storage for the last 40 years (and 4k for newer hardware). Linux won't even try to read a scsi disk with smaller sectors for example, and I doubt most other OSes will either. I would strongly recommend not changing this, higher or lower. It can only reduce hardware compatibility in exchange for the potential benefit of a small flash wear reduction. I would think it more important that a USB drive work universally than have marginally lower wear on a flash chip that should last hundreds of thousands of writes
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Yeah, windows doesn't support non-standard sector sizes either: https://learn.microsoft.com/en-us/troubleshoot/windows-server/backup-and-storage/support-policy-4k-sector-hard-drives
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Ok, will change to 512 or 4096 depending on the erase/write block size. Since most systems should support 4096 that seems like a relatively safe value, especially since the flash in microcontrollers seems to cap out at 4096 usually.
I would think it more important that a USB drive work universally than have marginally lower wear on a flash chip that should last hundreds of thousands of writes
10.000 erase cycles per page (erase block) in the case of the nrf52 and stm32l031 chips for example. It's quite low actually. Normally a flash translation layer is used to spread wear out over the entire chip to not destroy it by writing to the FAT or main directory over and over again (which might make it wear out after doing just 10.000 file operations).
...but it's not something we can do a whole lot about in the USB MSC driver except for avoiding write amplification.
aykevl
commented
Aug 27, 2025
Comment on lines
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| if uint32(blockStart)%m.blockSizeUSB == 0 { | ||
| firstBlock := uint32(blockStart) / uint32(m.dev.EraseBlockSize()) | ||
| numBlocks := (m.blockSizeUSB + uint32(m.dev.EraseBlockSize()) - 1) / uint32(m.dev.EraseBlockSize()) | ||
| m.dev.EraseBlocks(int64(firstBlock), int64(numBlocks)) | ||
| } |
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This makes the assumption that data is always written in full blocks, and that packets arrive sequentially. @mikesmitty can you confirm that this is true?
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That's how the protocol operates, yeah. A read or write command is sent from the host in a CDB packet along with the transfer length and then nothing but un-encapsulated raw data is sent/received until the transfer length is met
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16764 0 0.00% tinygo build -size short -o ./build/test.hex -target=pyportal -stack-size 8kb ./examples/net/http-get/ 119856 119856 0 0.00% 8020 8020 0 0.00% tinygo build -size short -o ./build/test.hex -target=arduino-nano33 -stack-size 8kb ./examples/net/tcpclient/ 308128 308128 0 0.00% 15924 15924 0 0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/websocket/dial/ 103568 103568 0 0.00% 10000 10000 0 0.00% tinygo build -size short -o ./build/test.hex -target=metro-m4-airlift -stack-size 8kb ./examples/net/socket/ 388388 388388 0 0.00% 18772 18772 0 0.00% tinygo build -size short -o ./build/test.hex -target=matrixportal-m4 -stack-size 8kb ./examples/net/webstatic/ 166364 166364 0 0.00% 9992 9992 0 0.00% tinygo build -size short -o ./build/test.hex -target=arduino-mkrwifi1010 -stack-size 8kb ./examples/net/tlsclient/ 155636 155636 0 0.00% 8812 8812 0 0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/mqttclient/natiu/ 116700 116700 0 0.00% 13296 13296 0 0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webclient/ 337948 337948 0 0.00% 21816 21816 0 0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webserver/ 338096 338096 0 0.00% 21740 21740 0 0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/mqttclient/paho/ 174508 174508 0 0.00% 13840 13840 0 0.00% tinygo build -size short -o ./build/test.hex -target=elecrow-rp2040 -stack-size 8kb ./examples/net/tlsclient/ 120168 120168 0 0.00% 11864 11864 0 0.00% tinygo build -size short -o ./build/test.hex -target=elecrow-rp2350 -stack-size 8kb ./examples/net/ntpclient/ 6251306 6251306 0 0.00% 963430 963430 0 0.00% |
Yeah, in the SCSI protocol read/write commands are specified by their starting block number and the length is given in number of full blocks. Transferring less than a full block would be expected to be discarded as incomplete. |
We could perhaps check whether they've been erased already (via unmap or something) but the default of most OSes seems to be to _not_ unmap blocks so we will have to erase in most cases anyway.
|
Updated the PR.
One thing I could change is to essentially add a new task specifically for erasing blocks, so that all blocks of a write operation are erased at once (this can be a bit faster than doing it block-by-block). But I guess the code as-is works fine.
Right, so erasing at the start of the write would be safe? Or is the device expected to buffer the entire block and only write it after it received the entire block of data? (Probably a moot point since any proper OS will write full blocks anyway). |
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We could perhaps check whether they've been erased already (via unmap or something) but the default of most OSes seems to be to not unmap blocks so we will have to erase in most cases anyway.
Tested on MacOS, and tested similar changes on Linux. They allow a FAT filesystem to be mounted and modified without causing problems due to a missing erase step.
@mikesmitty I haven't looked deeply into USB MSC or SCSI, from the behavior of MacOS at least it appears that it will always write full blocks. Do you know whether this is true generally, or only happens to be true in this case?