Grow UBI device?

[Wolfgang Denk]

Dear Artem,

In message <1381393038.7875.83.camel at sauron.fi.intel.com> you wrote:
> Wolfgang, people already answered you, and frankly I did not read all
> the answers :-) But here is my brief version.

Thanks.

> 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.

Correct.

> > 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.

Correct.

> > 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.

We don't.  We may want to do this later, but certainly not during this
migration steps.

> 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.

With JFFS2?  I would expect to see "UBIFS" here?

> 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.

Thanks for the poointer.  I would have missed this.

> 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.

Agreed.  It would have worked in our case, as mtd1 is 32 MB and mtd2
is 96 MB on a 128 MB NAND device, but it's much better to have full
control.

> 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.

The plan is to just have another mtd3 which covers the whole device.
So I would detach UBI from mtd2 and re-attach to mtd3.

> 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.

That's just amazing :-)

> 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.

Thanks a lot for all the help, it's highly appreciated!

Best regards,

Wolfgang Denk

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