When does sync not touch the db file?

steffi2 has asked for the wisdom of the Perl Monks concerning the following question:

I'm using dre.pl which is an old perlscript that sends RSS->email. It uses tie with DB_File to keep track of those links it's already seen in order to avoid them next time. Even though the code is touching the hash when it unties the db file's modified date is unchanged. Likewise if I can add an explicit $ref->sync() the db file's modified date remains unchanged. Are there reasons why the db file's modified date would remain unchanged even when you dirty the hash?

Comment on When does sync not touch the db file?

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Re: When does sync not touch the db file? by rjt (Curate) on Sep 08, 2013 at 18:59 UTC
What exactly do you mean by "touching the hash" and "dirty the hash"? `$db->sync` will not actually write anything unless the hash contents have actually been modified in some way (i.e., there must be some cached buffers in order to "Flush any cached buffers to disk"). You cannot force a write with `$db->sync`. For example: `$db->sync; # not modified $hash{foo} = 'bar'; # not modified $db->sync; # MODIFIED @hash{qw<one two three>} = 1..3; # not modified $db->sync; # MODIFIED delete $hash{two}; # not modified $db->sync; # MODIFIED $db->sync; # not modified` [download] Edit (Further thoughts): If you have definitely modified the hash and after a `$db->sync` the file still appears to be unmodified, you should be checking the return of `$db->sync`, which unfortunately returns `0` on success; if it returns `-1`, it failed, and a more detailed error is available in `$!`. `1` is also an error: "1 generally (but not always) means that the key specified did not exist in the database". Finally, remember by default stat and friends operate with whole-second resolution, so if you were to run my above code and use the file's mtime as your test, probably all lines would appear not modified. See stat (especially the caveats!), and consider using other metrics such as file size and Digest::MD5 checksum, but beware the latter may negatively impact performance if the file is large or checked very frequently. `use strict; use warnings;` omitted for brevity.	[reply] [d/l] [select]
Re: When does sync not touch the db file? by Random_Walk (Prior) on Sep 08, 2013 at 15:12 UTC
That sounds rather odd. Can you post a minimal fragment of code that exhibits this behaviour, along with details of your OS and Perl versions? Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply]