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From: Qu Wenruo <quwenruo.btrfs@gmx.com>
To: dsterba@suse.cz, Qu WenRuo <wqu@suse.com>,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH v2 0/7] btrfs-progs: Support for BG_TREE feature
Date: Tue, 22 Oct 2019 20:27:53 +0800
Message-ID: <bba76399-6b60-b3fb-afdd-74b8561dec1c@gmx.com> (raw)
In-Reply-To: <20191022122348.GV3001@twin.jikos.cz>

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On 2019/10/22 下午8:23, David Sterba wrote:
> On Tue, Oct 22, 2019 at 02:30:22PM +0800, Qu Wenruo wrote:
>> BTW, there is one important compatibility problem related to all the BGI
>> related features.
>>
>> Although I'm putting the BG_TREE feature as incompatible feature, but in
>> theory, it should be RO compatible.
> 
> It could be RO compatible yes.
> 
>> As except extent/bg tree, we *should* read the fs without any problem.
>>
>> But the problem is, current btrfs mount (including btrfs-check) still
>> need to go through the block group item search, even for permanent RO mount.
>>
>> This get my rescue mount option patchset to be involved.
>> If we have such skip_bg feature earlier, we can completely afford to
>> make all these potential features as RO compatible.
>>
>>
>> Now my question is,  should we put this feature still as incompatible
>> feature?
> 
> In some way it would probably have to be incompat, either full or RO. As
> unknown tree items are ignored, if the rest of the filesystem provides
> enough information to access the data, then incompat RO sounds like the
> best option. And that's probably independent of how exactly the new BGI
> is done.
> 
But the problem is, older fs can't mount it at all, no matter RO or RW.

It's now completely dependent on how older kernel handles missing block
group items.

If current kernel has something like skip_bg or fall back to RO +
skip_bg by default, then it's really RO compatible.
But we all know how picky current btrfs is about missing block group
items...

So, I guess we will have a RO compatible feature that can't really be
mounted RO by older kernel at last?

Thanks,
Qu


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      reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-08  4:49 Qu Wenruo
2019-10-08  4:49 ` [PATCH v2 1/7] btrfs-progs: Refactor excluded extent functions to use fs_info Qu Wenruo
2019-10-08  9:22   ` Johannes Thumshirn
2019-10-17  2:16   ` Anand Jain
2019-10-08  4:49 ` [PATCH v2 2/7] btrfs-progs: Refactor btrfs_read_block_groups() Qu Wenruo
2019-10-17  3:23   ` Anand Jain
2019-10-17  4:33     ` Qu Wenruo
2019-10-17  5:08       ` Anand Jain
2019-10-08  4:49 ` [PATCH v2 3/7] btrfs-progs: Enable read-write ability for 'bg_tree' feature Qu Wenruo
2019-10-17  4:56   ` Anand Jain
2019-10-08  4:49 ` [PATCH v2 4/7] btrfs-progs: mkfs: Introduce -O bg-tree Qu Wenruo
2019-10-08  8:16   ` [PATCH v2.1 " Qu Wenruo
2019-10-08  4:49 ` [PATCH v2 5/7] btrfs-progs: dump-tree/dump-super: Introduce support for bg tree Qu Wenruo
2019-10-08  4:49 ` [PATCH v2 6/7] btrfs-progs: check: Introduce support for bg-tree feature Qu Wenruo
2019-10-08  4:49 ` [PATCH v2 7/7] btrfs-progs: btrfstune: Allow to enable bg-tree feature offline Qu Wenruo
2019-10-17  4:17   ` Anand Jain
2019-10-17  4:28     ` Qu Wenruo
2019-10-14 15:17 ` [PATCH v2 0/7] btrfs-progs: Support for BG_TREE feature David Sterba
2019-10-15  0:32   ` Qu Wenruo
2019-10-16 11:16     ` David Sterba
2019-10-16 11:19       ` Qu WenRuo
2019-10-18 17:27         ` David Sterba
2019-10-19  0:04           ` Qu Wenruo
2019-10-21 15:44             ` David Sterba
2019-10-22  0:49               ` Qu Wenruo
2019-10-22  6:30                 ` Qu Wenruo
2019-10-22 12:23                   ` David Sterba
2019-10-22 12:27                     ` Qu Wenruo [this message]

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