Retrieving number of free unused eraseblocks in a UBI filesystem

[Martin Townsend]

On Wed, Dec 7, 2016 at 2:17 PM, Boris Brezillon
<boris.brezillon at free-electrons.com> wrote:
> On Wed, 7 Dec 2016 13:55:42 +0000
> Martin Townsend <mtownsend1973 at gmail.com> wrote:
>
>> On Wed, Dec 7, 2016 at 1:14 PM, Boris Brezillon
>> <boris.brezillon at free-electrons.com> wrote:
>> > On Wed, 7 Dec 2016 13:07:25 +0000
>> > Martin Townsend <mtownsend1973 at gmail.com> wrote:
>> >
>> >> On Wed, Dec 7, 2016 at 12:39 PM, Boris Brezillon
>> >> <boris.brezillon at free-electrons.com> wrote:
>> >> > On Wed, 7 Dec 2016 11:59:13 +0000
>> >> > Martin Townsend <mtownsend1973 at gmail.com> wrote:
>> >> >
>> >> >> Hi,
>> >> >>
>> >> >> I'm running a 4.1 Kernel and have a UBI Filesystem with 2 volumes
>> >> >> taking up all the NAND.
>> >> >> ubinfo -d 0
>> >> >> ubi0
>> >> >> Volumes count:                           2
>> >> >> Logical eraseblock size:                 126976 bytes, 124.0 KiB
>> >> >> Total amount of logical eraseblocks:     4016 (509935616 bytes, 486.3 MiB)
>> >> >> Amount of available logical eraseblocks: 0 (0 bytes)
>> >> >> Maximum count of volumes                 128
>> >> >> Count of bad physical eraseblocks:       56
>> >> >> Count of reserved physical eraseblocks:  24
>> >> >> Current maximum erase counter value:     15
>> >> >> Minimum input/output unit size:          2048 bytes
>> >> >> Character device major/minor:            249:0
>> >> >> Present volumes:                         0, 1
>> >> >>
>> >> >> So I'm guessing that the Total amount of logical erase blocks is 0 as
>> >> >> this is because I have 2 volumes of a fixed size that take up all the
>> >> >> available eraseblocks of the /dev/ubi0 device.
>> >> >>
>> >> >> Is there a way of getting, per volume preferably, the number of
>> >> >> eraseblocks that aren't being used? Or conversely get the number of
>> >> >> eraseblocks that are used as I can work it out from the total amount
>> >> >> of logical eraseblocks.
>> >> >
>> >> > cat /sys/class/ubi/ubiX_Y/reserved_ebs
>> >> >
>> >> > where X is the UBI device id and Y is the volume id.
>> >> >
>> >>
>> >> Thanks for the swift reply, this seems to give me the total number of
>> >> eraseblocks:
>> >>
>> >> # cat /sys/class/ubi/ubi0_1/reserved_ebs
>> >> 3196
>> >>
>> >> # ubinfo -d0 -n1
>> >> Volume ID:   1 (on ubi0)
>> >> Type:        dynamic
>> >> Alignment:   1
>> >> Size:        3196 LEBs (405815296 bytes, 387.0 MiB)
>> >> State:       OK
>> >> Name:        rootfs
>> >>
>> >> # df -h .
>> >> Filesystem      Size  Used Avail Use% Mounted on
>> >> ubi0:rootfs     357M  135M  222M  38% /
>> >>
>> >> What I would like to know is how many of them are being used and how
>> >> many are not.
>> >
>> > Well, for static volumes, you could extract this information from the
>> > data_bytes sysfs file. It doesn't work for dynamic volumes though, but
>> > even if you could extract this information from the number of mapped
>> > LEBs, not sure it would have any meaning, because unmapped LEBs can
>> > still be reserved by the upper layer, and just because you have a lot
>> > of unmapped LEBs does not mean you can shrink the volume (which I guess
>> > is your ultimate goal).
>> >
>>
>> My goal is for failover, the SOM we use has 2 flash devices, the
>> primary is UBI on NAND, which we keep in sync with the other flash
>> device which is eMMC.  I'm basically monitoring both erase counts
>> using Richard's ubihealthd patch and bad blocks as an indication of
>> when to failover.  Maybe naively I'm thinking if I use the number of
>> unused eraseblocks it could serve 2 purposes failover due to badblocks
>> which will decrease available blocks naturally and also fail over to
>> the larger secondary flash device if it becomes near full.
>>
>> If I stick to using bad blocks as a metric can I safely assume that
>> every time a block is marked bad it will decrease the overall size of
>> the filesystem as seen by utilities such as 'df'?
>
> That's not the case. UBI reserves some amount of blocks to deal with
> bad blocks, which means your FS available size will not decrease, or at
> least, not until you are in a situation where you run out of erase
> blocks reserved for bad block handling. And even in that case, assuming
> the UBI device still has some blocks available (i.e. not reserved for a
> volume or for internal UBI usage),  those blocks will be used when a
> new bad block is discovered.

Thanks for this it clears up a misconception I had, I was assuming it
would take blocks from those allocated to the volumes which is not the
case.  So if I reserve say 50 blocks for bad block management and then
split the remaining blocks between the 2 volumes and then once 50
blocks have gone bad, the next bad block will result in a corrupt
filesystem?

>
>> If so maybe it's
>> best if I stick to a combination of number of bad blocks from MTD in
>> sysfs and free space in filesystem.
>
> Not sure free FS space is a good metric either. Maybe you should just
> monitor the number of available blocks and the number of blocks
> reserved for bad block handling at the UBI level (not sure those
> numbers are exposed through sysfs though). When these numbers are too
> low, you should consider switching to the other storage.

I'm having a dig through the code and if I expose used_ebs from struct
ubi_volume to sysfs would this give me what I want, the comment say
"how many logical eraseblocks in this volume contain data" which looks
like exactly what I want.  I could then monitor the bad blocks and if
they get to the reserved bad block amount fail over and then use
used_ebs to see if it gets within a % of the "reserved_ebs" value from
sysfs?