Suggestion needed to set proper journal size

[Artem Bityutskiy]

On Fri, 2012-04-20 at 15:59 -0700, Subodh Nijsure wrote:
> Hello,
> 
> I know general implications of changing journal size on UBIFS -- bigger 
> the journal,
> longer it may take to replay the journal, increasing time UBIFS takes to 
> mount. Smaller
> journal means more re-writes to flash.
> 
> We created 2 partitions 100MB and 45MB and we see that by default 
> mkfs.ubifs is reserving 8MB for journal.
> 
> UBIFS: file system size:   101834752 bytes (99448 KiB, 97 MiB, 802 LEBs)
> UBIFS: journal size:         9023488 bytes (8812 KiB, 8 MiB, 72 LEBs)
> 
> UBIFS: file system size:     47996928 bytes (46872 KiB, 45 MiB, 378 LEBs)
> UBIFS: journal size:          9023488 bytes (8812 KiB, 8 MiB, 72 LEBs)
> 
> I know I can specify size of journal with -j option.
> 
> Should the journal size be scaled in some proportion of size of UBIFS 
> partition?

It does grow, but not infinitely. We have some heuristics in mkfs.ubifs,
but those were mostly taken out of thin air. I do not remember us doing
a lot of experimenting with that.

> Should the journal size depend on how often you expect file-system to be 
> updated?
> 
> Has anyone done experiments with setting really small (1MB/2MB) journal 
> size?
> What side effects is one likely to encounter if the journal size is too 
> small.
> 
> fwiw in my use case, file-system doesn't get updated that often, perhaps 
> 2K data in about 4 files once a week.

I think you should do your own experiments. If you share the results -
will be nice.

-- 
Best Regards,
Artem Bityutskiy
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