Resizing of an existing UBIFS

[Ricard Wanderlof]

On Mon, 28 May 2012, Adrian Hunter wrote:

> On 25/05/12 11:00, Ricard Wanderlof wrote:
>>
>> On Fri, 25 May 2012, Robert Homann wrote:
>>
>>> It is definitely a lot simpler to just set this parameter to some big
>>> value and generate new images from scratch since the overhead in space
>>> consumption is not an issue in this case. But let's say, in my case the
>>> purpose of using such a resizing tool is not space optimization, but
>>> fixing a former decision...
>>
>> I think this is a most important point. Whatever sizing strategy and volume
>> sizes that are decided upon, one can be sure that a couple of years down the
>> line differing circumstances will require a re-think of that strategy, no
>> matter how well founded the decision was to start with.
>
> Unless you set max_leb_cnt to match the media size.
>
> The point is resizing when max_leb_cnt is not changed is much simpler.

One point I ran into though is that when specifying a larger max-leb-cnt 
than the size of the volume, by default the journal size also increases
(it is according to Artem 12% of the max-leb-cnt 
(http://lists.infradead.org/pipermail/linux-mtd/2012-March/040084.html), 
but on my system it seems to be less than that and varies with the LEB 
count; see below - perhaps 12% is the maximum size?).

I've got a 128 MB flash with 128 kB blocks on my system, i.e. a LEB size 
of 129024 and a total LEB count for the whole device of 1024.

Say you have an ~8 MB volume on the flash = 65 LEBs, and you create a file 
system image for it with a max-leb-cnt of 65. This results in a default 
journal size of 903168 bytes according to mkfs.ubifs -v. (Incidentally, 
the journal size here appears to be about 11% of the total LEB count for 
the volume.)

If you now create a file system image with a max-leb-cnt of 1024, this 
results in a default journal size of 8388608 = 8M which is larger than the 
space available in the volume. (Incidentally, the journal size in this 
case appears to be about 6% of the total LEB count specified).

Of course, the volume size could be set to, say, 16 MB instead, but 
neverthless, 8 MB of this 16 MB volume is then wasted for a journal 
intended to fit a (future) larger volume size.

So, just increasing max-leb-cnt causes a lot of wasted space in a small 
volume by default. Of course one could specify a smaller journal size, the 
question is then what journal size to set? Are there any guidelines which 
would be helpful in this case (Artem?) ?

/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