Re: [PATCH v5 9/9] btrfs: btrfs_copy_file_range() only supports reflinks

From: Christoph Hellwig <hch@infradead.org>
To: P??draig Brady <P@draigBrady.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Anna Schumaker <Anna.Schumaker@netapp.com>,
	linux-nfs@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
	zab@zabbo.net, viro@zeniv.linux.org.uk, clm@fb.com,
	darrick.wong@oracle.com, mtk.manpages@gmail.com,
	andros@netapp.com
Subject: Re: [PATCH v5 9/9] btrfs: btrfs_copy_file_range() only supports reflinks
Date: Mon, 12 Oct 2015 07:34:44 -0700	[thread overview]
Message-ID: <20151012143444.GA10156@infradead.org> (raw)
In-Reply-To: <561B8A09.5070507@draigBrady.com>

On Mon, Oct 12, 2015 at 11:23:05AM +0100, P??draig Brady wrote:
> You're right that if the user doesn't notice, then there is no
> point exposing this. However I think the user does notice as
> there is a difference in the end state of the copy.  I.E. generally
> if there is a different end state it would require an option,
> while if only a different copying mechanism it would not.
> I think the different end state of a reflink warrants an option for 3 reasons:
> 
>  - The user might want separate bits for resiliency. Now this is
>    a weak argument due to possible deduplication in lower layers,
>    but still valid is some setups.

This one is completely bogus.  For one because literally every lower
layer can and increasinly will dedup or share in some form.  If we
prentend we could do this we actively mislead the user.

>  - The user might want to avoid CoW at a later time critical stage.
> 
>  - The user might want to avoid ENOSPC at a later critical stage.

These two are the same and would be the argument for the "falloc" flag
I mention before.  But we'd need to sit down and specify the exact
semantics for it.  For example one important question that comes to mind
is if it also applies for extents that are holes in the source range.

I'd much rather get the basic system call in ASAP and then let people
explain their use cases for this and only add it once we've made sure
we have consistent semantics that actually fit the users needs.