Category Archives: Software

CRC

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I watched Ben Eater’s videos How do CRCs work? and Checksums and Hamming distance (the full course is Learn about error detection) then found the Wikipedia page: Cyclic redundancy check.

Also of interest:

Data corruption

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Oh, yuck. ZFS-8000-8A. :(

-------------------
Mon May 29 16:23:47 [bash:5.2.15 jobs:0 error:0 time:35]
root@charm:/home/jj5
# zpool status -v
  pool: fast
 state: ONLINE
status: One or more devices has experienced an error resulting in data
        corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
        entire pool from backup.
   see: https://openzfs.github.io/openzfs-docs/msg/ZFS-8000-8A
config:

        NAME                                              STATE     READ WRITE CKSUM
        fast                                              ONLINE       0     0     0
          mirror-0                                        ONLINE       0     0     0
            nvme-Samsung_SSD_990_PRO_2TB_S6Z2NJ0W215171W  ONLINE       0     0     2
            nvme-Samsung_SSD_990_PRO_2TB_S6Z2NJ0W215164J  ONLINE       0     0     2

errors: Permanent errors have been detected in the following files:

        /fast/vbox/218-jj-wrk-8-charm-prod-vbox/218-jj-wrk-8-charm-prod-vbox.vdi
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Target then source

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I’m surprised it took me this long to figure this out. But there’s a very big problem with a command like this:

rsync var/data/ target:/var/data/

The problem is that in order to type that command, you first have to type the first part of the command, which includes this command:

rsync var/data/ target:/

That first part of the command is in fact a valid command, and if you pressed ENTER accidentally before you had finished typing the whole command, then rsync would begin to replace your entire root file system with the contents of var. That’s the sort of thing that will ruin your day.

So from now on, when I write command-line tools, I will nominate the target *before* the source, and if both the target and source are not specified then I will return an error.

p.s. Yes, the reverse problem exists, that when you nominate the target first you can still fuck up with the source, but given that you are operating on the target from the source, if you get the source wrong, you can just fix your command and run it again and it will be fixed up.