Re: Speeding up slooooowwww extractions

[Graham Percival <gperciva@tarsnap.com>]

On Thu, May 27, 2021 at 01:11:03AM +0200, hvjunk wrote:
>  Another “Request” - File Checksums (SHA256/etc.) in the ‘-tv’ (or -tvv)
>  output - that way I can check whether the backed up / stored file in the
>  archive is similar to the one on disk - especially usefull when doing
>  things like the above, and then with a -tvvv each block’s checksum

File checksums would be tricky, since Tarsnap stores chunks rather than files.
But I have some experimental code from a few years ago which adds something
similar to this for creating an archive (potentially with --dryrun).

    --probability-check-file p
    (c mode only) If a file has not changed, read it anyway (with the given
    probability p from 0.0 to 1.0) and compare its hash(es) and trailer to the
    cached values.  This might provide a warning about a failing disk if other
    detection methods fail.  However, this operation will make Tarsnap run
    considerably slower, so we do not recommend using a high probability with
    large archives.

There were a few details in that code which needed to be cleaned up.

- Graham