linux-kernel - Re: [PATCH] btrfs: also add stripe entries for NOCOW writes

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Message-ID: <d42756f6-d5a8-4f44-a6f0-6056f5c1015b@gmx.com>
Date: Mon, 23 Sep 2024 17:26:39 +0930
From: Qu Wenruo <quwenruo.btrfs@....com>
To: Johannes Thumshirn <Johannes.Thumshirn@....com>,
 Johannes Thumshirn <jth@...nel.org>, Chris Mason <clm@...com>,
 Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
 "open list:BTRFS FILE SYSTEM" <linux-btrfs@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>
Cc: WenRuo Qu <wqu@...e.com>, Naohiro Aota <Naohiro.Aota@....com>
Subject: Re: [PATCH] btrfs: also add stripe entries for NOCOW writes



在 2024/9/23 17:10, Johannes Thumshirn 写道:
> On 23.09.24 09:28, Qu Wenruo wrote:
>>
>>
>> 在 2024/9/23 16:15, Johannes Thumshirn 写道:
>>> From: Johannes Thumshirn <johannes.thumshirn@....com>
>>>
>>> NOCOW writes do not generate stripe_extent entries in the RAID stripe
>>> tree, as the RAID stripe-tree feature initially was designed with a
>>> zoned filesystem in mind and on a zoned filesystem, we do not allow NOCOW
>>> writes. But the RAID stripe-tree feature is independent from the zoned
>>> feature, so we must also allow NOCOW writes for zoned filesystems.
>>>
>>> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
>>
>> Sorry I'm going to repeat myself again, I still believe if we insert an
>> RST entry at falloc() time, it will be more consistent with the non-RST
>> code.
>>
>> Yes, I known preallocated space will not need any read nor search RST
>> entry, and we just fill the page cache with zero at read time.
>>
>> But the point of proper (not just dummy) RST entry for the whole
>> preallocated space is, we do not need to touch the RST entry anymore for
>> NOCOW/PREALLOCATED write at all.
>>
>> This makes the RST NOCOW/PREALLOC writes behavior to align with the
>> non-RST code, which doesn't update any extent item, but only modify the
>> file extent for PREALLOC writes.
>
> Please re-read the patch. This is not a dummy RST entry but a real RST
> entry for NOCOW writes.
>
I know, but my point is, if the RST entry for preallocated range is
already a regular one, you won't even need to insert/update the RST tree
at all.

Just like we do not need to update the extent tree for
NOCOW/PREALLOCATED writes.

Thanks,
Qu