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List:       util-linux-ng
Subject:    Re: [PATCH V2] libblkid: add support for UBI superblock
From:       Rafał Miłecki <zajec5 () gmail ! com>
Date:       2017-08-07 8:21:55
Message-ID: CACna6rz-1CAeLWTB69OQ=219ZjX7f1uzQmeXhfYxd5qfC9m=qQ () mail ! gmail ! com
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On 7 August 2017 at 10:18, Karel Zak <kzak@redhat.com> wrote:
> On Mon, Aug 07, 2017 at 07:40:17AM +0200, Rafał Miłecki wrote:
>> On 3 August 2017 at 14:26, Karel Zak <kzak@redhat.com> wrote:
>> > On Wed, Aug 02, 2017 at 02:14:18PM +0200, Rafał Miłecki wrote:
>> >> From: Rafał Miłecki <rafal@milecki.pl>
>> >>
>> >> UBI is a volume management system that can be used on a raw flash
>> >> partition for providing multiple logical volumes. Detecting UBI
>> >> superblock may be useful for tools wanting to simplify or automate
>> >> attaching UBI.
>> >>
>> >> Please note it's not directly related to the ubifs support which is just
>> >> a filesystem working on top of UBI volume.
>> >>
>> >> In other words: UBI can be used on MTD partition (e.g. /dev/mtdblock0)
>> >> while ubifs can be used on UBI volume (e.g. /dev/ubi0_0).
>> >>
>> >> This patch adds simple code reading UBI version and unique number and
>> >> setting it in the blkid_probe.
>> >>
>> >> Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
>> >> ---
>> >> V2: Update commit message (more UBI vs. ubifs details)
>> >>     Pass &hdr->image_seq to the blkid_probe_sprintf_uuid
>> >>     Add test
>> >
>> > Applied, thanks.
>> >
>> >> +ID_FS_TYPE=ubi
>> >> +ID_FS_USAGE=filesystem
>> >
>> > This is probably not ideal. We use "RAID" flag for logic volume
>> > managers. The advantage is that probing is faster as it does not try
>> > to check for another signature when "RAID" is detected.
>> >
>> > The question is how small could be the UBI device? We don't probe for
>> > "RAID" on very small devices (<=1MiB).
>>
>> UBI is responsible for bad blocks handling, so it needs enough erase
>> blocks (PEBs) reserved in order to work. If one's MTD partition is as
>> small as 1 MiB, it won't have enough of erase blocks (PEBs) [0].
>>
>> There is some formula for amount of reserved blocks, like:
>> n_reserved = n_max_bad_blocks + n_internal_volumes_blocks  + 1
>> but basically 1 MiB is said to be too small.
>>
>> Does it mean we should & we're save to switch from OTHER to the RAID?
>
> I have pushed this change to the master branch. Now UBI uses
> BLKID_USAGE_RAID flag (and the test image has been enlarged (> 1MiB)).

Thank you! I appreciate your help on getting this feature in place :)

-- 
Rafał
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