Re: [PATCH 04/11] fs: add support for allowing applications to pass in write life time hints

From: Jens Axboe <axboe@kernel.dk>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	adilger@dilger.ca, martin.petersen@oracle.com
Subject: Re: [PATCH 04/11] fs: add support for allowing applications to pass in write life time hints
Date: Mon, 19 Jun 2017 10:02:09 -0600	[thread overview]
Message-ID: <8db675e7-b868-bae1-784a-33cba67d0874@kernel.dk> (raw)
In-Reply-To: <b04642e6-4ecc-f2e8-8184-6fff302d3d0b@kernel.dk>

On 06/19/2017 08:56 AM, Jens Axboe wrote:
> On 06/19/2017 12:27 AM, Christoph Hellwig wrote:
>> On Sat, Jun 17, 2017 at 01:59:47PM -0600, Jens Axboe wrote:
>>> Add four flags for the pwritev2(2) system call, allowing an application
>>> to give the kernel a hint about what on-media life times can be
>>> expected from a given write.
>>>
>>> The intent is for these values to be relative to each other, no
>>> absolute meaning should be attached to these flag names.
>>>
>>> Set aside 3 bits in the iocb flags structure to carry this information
>>> over from the pwritev2 RWF_WRITE_LIFE_* flags.
>>
>> What is the strong use case for the per-I/O flags?  I'd much rather
>> stick to fcntl only for now if we can.
> 
> Fine, I guess I should just have dusted off the 2 year old patchset,
> as that was _exactly_ what that did.

Actually, one good use case is O_DIRECT on a block device. Since I'm
not a huge fan of having per-call hints that is only useful for a
single case, how about we add the hints to the struct file as well?
For buffered IO, just grab it from the inode. If we have a file
available, then that overrides the per-inode setting.

-- 
Jens Axboe