Resizing of an existing UBIFS

[Ricard Wanderlof]

On Fri, 1 Jun 2012, Adrian Hunter wrote:

> If you let mkfs.ubifs calculate the journal size it will limit it to 8MiB max.

Thanks, that explains the figures then.

> The journal must be scanned after an unclean unmount (e.g. power loss)
> so a small journal is better for that.  However once the journal is full it
> must be committed which involves a certain amount of overhead so a bigger
> journal is better for that, but at some point it makes hardly any difference.
>
> You can use the flash read speed to get an idea of how big a journal you can
> live with. e.g. in the worst case you want to mount in 3 seconds, your read
> speed is 4MiB/s so you need a journal less than 12MiB (but reading the
> journal is just one part of mounting so maybe 8MiB is better).

So what you're saying is that basically it's the worst-case mount time 
that together with the read speed sets the maximum journal size? I assume 
if the unmount was clean there is no need to read the journal, or does it 
need to be scanned anyway? What if there was an unclean unmount but no 
data recently written so nothing in the journal, does it still have to be 
scanned in full?

> You can use the flash write speed to get an idea of how often a commit
> happens.  e.g. flash write speed is 3MiB/s, journal is 4MiB, a commit begins
> when the journal is 13/16 full, so there is a commit at most once per
> second, which sounds OK.

In my case there's not a lot of writes (much less than the flash write 
speed would make possible), so even a small journal would probably be ok. 
Even if there were the odd burst of writes we could live with a temporary 
surge in flash writes.

One question here: when one mounts an empty volume, it seems the ubifs 
creates a journal with some default value (which doesn't seem to be 
identical to the defaults that mkfs.ubifs uses). Is there any way to 
adjust those defaults, either with mount command options or with the 
sysfs?

What I'm getting at in a more concrete sense, is that if I create an empty 
8 MB UBI volume, then mount it as a ubifs file system (rather than 
mkfs.ubifs'ing an empty directory, setting appropriate parameters on the 
mkfs.ubifs command line, and ubiupdatevol'ing that), can I specify a 
maximum LEB count (and fix the journal size) that would be appropriate 
also for a future larger volume size?

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30