ZEO cache best practices, was Re: [ZODB-Dev] ZEO 3.2 (Zope 2.7) ->3.6 (Zope 2.9) upgrading: Much slower startup due to cache file creation

[Jim Fulton]

Tim Peters wrote:
> [Paul Winkler]
> 
>>Heh. Well, I had a site that under some usage patterns would
>>occasionally slow to a crawl with cache flips every few minutes.  That
>>was with the old default 20 MB cache size.  I think I left it at
>>500 MB or so and that site's been fine since. But the performance
>>demands were pretty low.
> 
> 
> Yes, 20MB is _very_ small.  It may have seemed conservatively safe 10
> years ago when disks were much smaller, but now it's ludicrously
> small.
> 
> OTOH, the larger the ZEO cache file, the longer it may take startup or
> reconnection cache verification to complete, so there's always some
> reason not to do the obvious thing <0.3 wink>.

This is only a problem if:

1. a persistent cache is used or

2. the client gets disconnected from the server, without
    restarting, long enough for the server to commit enough
    transactions for the server to be able to do enough quick
    verification.

We seem to have a lot of problems with persistent caches for
some reason, so I tend to recommend against their use.  I'm
not sure what's going on there.  I still find them useful in
situations in which the connection to the server is slow.

the second case should be rare.

I would definately error in the direction of using a
larger cache

Jim

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