Grow UBI device?

[Artem Bityutskiy]

Wolfgang, people already answered you, and frankly I did not read all
the answers :-) But here is my brief version.

On Tue, 2013-10-08 at 21:28 +0200, Wolfgang Denk wrote:
> I'm looking for a clever way to migrate devices (in the field) from
> JFFS2 to UBI/UBIFS.  In the end result, I want to have single UBI
> device (which covers the whole NAND chip) with several volumes used
> with UBIFS.  Unfortunately I must keep some of the existing data.

And I guess there is no temporary storage where you could copy JFFS2
files, re-format the flash, and copy the files back.

> It would be nice if I could start with a UBI device that is smaller
> than the NAND, so I can keep an existing MTD partition with a JFFS2
> file system.

You mean that on the existing systems, you already have an extra MTD
partition, or you somehow can create one.

Then you want to UBI/UBIFS-format the extra partition. Then copy
required JFFS2 files there. And then kill JFFS2 partitions and "merge"
them with the extra partition, and end up with UBI/UBIFS covering all
the space.

>  After copying the data from JFFS2 to a UBIFS volume, I
> would like to free and reuse the space of this JFFS2 partition.

OK, sounds like I understood you correctly.

> Is there a way to "grow" the existing UBI device so that it now also
> covers the rest of the NAND chip?  Or is my only option to create a
> second UBI device?

I think so, unless you use the fastmap feature. Suppose you don't.

There are 2 parts - UBI and UBIFS.

So, let's suppose your situation is like this.

1. You have 2 mtd partitions - mtd1 and mtd2.
2. mtd1 has JFFS2 with important files.
3. mtd2 is the extra partition you have.

I guess you can do this.

1. Format mtd2 with UBI. Just erasing should be OK, or you can use
ubiformat: http://www.linux-mtd.infradead.org/faq/ubi.html#L_ubierase

Then you attach mtd2 (ubiattach) and you get an UBI device (/dev/ubiX).

2. Create a volume on /dev/ubiX which spans entire UBI device
(ubimkvol -t AFAIR). You end up with UBI volume /dev/ubiX_0.

3. And here is important part - you format /dev/ubiX_0 with JFFS2 but
correctly.

UBIFS has this "automatic expand" feature. Which means that if the UBI
volume underneath it becomes larger, UBIFS will expand automatically and
take the entire (larger) UBI volume.

However, there is a _limit_ to how much UBIFS can expand from the
original size. And you want to watch this limit and make it big enough.

The limit is connected with the "-c" option of mkfs.ubifs. Please, check
the docs - they explain what exactly -c controls and how it is related
to the maximum UBIFS file-system size:

http://www.linux-mtd.infradead.org/faq/ubifs.html#L_max_leb_cnt

So, select proper -c.

If you do not use mkfs.ubifs, but just attach /dev/ubiX_0, the UBIFS
kernel driver will format the volume automatically. It will select the
-c value itself. Since this value is power of 2, it will just take the
smallest power-of-2 value. It may be OK for your purposes, may be not.
It is safer to use mkfs.ubifs with explicit -c.

Basically this is the trickiest part, rest _should_ be easy, but you'll
learn when you try :-)

4. Copy important files from JFFS2-mtd1 to UBIFS-UBI-mtd2.

5. Erase mtd1 (or you can do this later too)

6. I do not know how you do this, but kill mtd1, and resize mtd2.
Probably mtd2 will then be named mtd1 after reboot. If you always refer
your MTD partitions and UBI volumes by name, you should not suffer to
much from mtd2 becoming mtd1.

Lets suppose there is no rename, for simplicity.

7. Make sure former JFFS2-mtd1 eraseblocks are erased (unless you did
this at step 5).

8. Attach mtd2 to UBI. UBI will automatically "take" the new
eraseblocks, erase them and write UBI headers there, and make available.

9. Volume size will still be the same, and you need to re-size it using
ubiresize.

10. Mount the resized volume. UBI will enlarge itself.

Problem solved.

Surely you do not want power cuts during this operations. If you need to
survive power cuts, I guess it is possible, but more complex.

HTH.

-- 
Best Regards,
Artem Bityutskiy