[Dirvish] A suggestion for dirvish-expire

[Kenneth Lerman]

If you build it, they will come.

Just write your own and give it a different name (dirvish-safe-expire 
might work).

Ken

On 12/4/2012 2:44 PM, Hernán J. González wrote:
> I've using Dirvish for a long time, and I really like it. Thanks.
>
> A suggestion for dirvish-expire, which current behaviour I consider
> rigid and very dangerous.
>
> Currently, the way of specifying which backups should be trimmed
> (expired) is via the crontab-like "expire-rule" entries.
> That seems quite dangerous and difficult to maintain. A crontab spec
> is good for events in the future, not in the past.
>
> In most  typical scenarios, what the user wants is to keep all or
> nearly all the recent backups, and a few of the older ones. That is,
> we want to have high temporal granularity for recent backups and low
> granularity for older ones, just like many apps do (MRTG).
>
> Currently, that's difficult to specify, and much more difficult to change.
> Suppose I run a full dirvish on Monday-Wednesday-Friday
> I could specify my expire rules like these:
>
> expire-rule:
>       *     *     *     *           *    +10 days
>       *     *     *     *           5    +2 months
>       *     *     1-7   *           5    +9 months
>       *     *     1-7   1,5,9       5    +4 years
>
> This means:
> - keep the file for 4 years if first friday of months Jan/May/Sep (3
> backups per year)
> or
> - keep the file for 9 months if first friday of any month (1 backup per month)
> or
> - keep the file for 2 months if friday  (1 backup per week)
> or
> - keep all files younger than 10 days
>
> This works, and might seem ingenious. Actually, it's terribly fragile,
> it's a nightmare to maintain - or a bomb waiting to explode
> Suppose someday you want to change your backup schedule, and you start
> running dirvish on Sunday-Tuesday-Thrusday. You're doomed. You can't do it.
> And if you do it and naively change the '5' (week day) in
> your expire-rules to '4' ... ooops... you've lost all your old backups.
> (And if you don't change them, the new backups will expire after 10 days)
>
> I think this is unacceptable.
>
> I propose then to change dirvish-expire so that it bases on more
> natural rules, eg
>
> age-interval:
>   +4 y:   4 m
>   +9 m:   1 m
>   +2 m:   1 w
>
> which means: for backups older than 4 years, keep at most 1 backup
> each 4 months;
> for backups older than 4 months, keep at most 1 backup each 1month; etc
> The expire algorithm would start from the most recent backup and trim
> the next backups
> that are separated from the current one in less than the prescribed interval.
> This is conceptually clear and has no dangers
> (I've done some similar perl scripts for trimming some old-style
> -tarballs- backups)
>
> Another useful (orthogonal) option to dirvish-expire would be to
> look for some special file inside the vault that works as a "dont ever
> expire me" flag.
>
>
> Hernán J. González
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