> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Gary Mills
> There's actually no such thing as bitrot on a disk.  Each sector on
> the disk is accompanied by a CRC that's verified by the disk
> controller on each read.  It will either return correct data or report
> an unreadable sector.  There's nothing inbetween.

There is something in between:  Corrupt data that happens to pass the
hardware checksum.  You said CRC, but CRC is a specific algorithm.  All the
various disk manufacturers can implement various different algorithms,
including parity, CRC, LDPC, or whatever.

Of all the algorithms they use inside the actual disk, CRC would be a
relatively strong one.  And if there was "nothing in between" absolute
accuracy and absolute error...  Then there would be no point in all the
stronger checksum algorithms, such as SHA256.  There would be no point for
ZFS to bother doing checksumming, if the disks could never silently return
corrupted data.

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