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

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Message-ID: <3c0c8517-a642-4e7b-bbcd-ef0022c49c3f@wdc.com>
Date: Mon, 23 Sep 2024 07:40:56 +0000
From: Johannes Thumshirn <Johannes.Thumshirn@....com>
To: Qu Wenruo <quwenruo.btrfs@....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

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.