Retrieving number of free unused eraseblocks in a UBI filesystem

[Martin Townsend]

On Wed, Dec 7, 2016 at 3:00 PM, Boris Brezillon
<boris.brezillon at free-electrons.com> wrote:
> On Wed, 7 Dec 2016 14:47:13 +0000
> Martin Townsend <mtownsend1973 at gmail.com> wrote:
>
>> 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?
>
> Hm, I'm not sure it will directly result in a FS corruption, since some
> blocks might actually be available at the time the FS tries to map a
> LEB. But it's likely to result in a UBI/UBIFS error at some point (when
> your FS is full and UBI fails to pick a PEB for a new LEB).
>
>>
>> >
>> >> 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.
>
> ->used_ebs is always set to ->reserved_pebs for dynamic volumes, so no.
>
>> 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?
>
> IMO, you should just rely on information exposed at the UBI device
> level (/sys/class/ubi/ubiX/).
>
> You already have 'reserved_for_bad' and 'avail_eraseblocks' exposed,
> and this should be enough. When reserved_for_bad approaches 0, then you
> should be near the EOL of your UBI device.

Thanks for the info. I get the bad blocks now, and can see that
monitoring 'reserved_for_bad' for 0 or close to 0 is one trigger.
The 'avail_eraseblocks' is always 0 on my board and it has around 60%
free space, I suppose this means that every block contains some
filesystem data, ie with lots of small files the erase blocks get used
up?