linux-kernel - Re: [PATCH] io_uring: add support for barrier fsync

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Date:   Tue, 9 Apr 2019 18:56:56 +0000
From:   Chris Mason <clm@...com>
To:     Jens Axboe <axboe@...nel.dk>
CC:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] io_uring: add support for barrier fsync

On 9 Apr 2019, at 14:46, Jens Axboe wrote:

> On 4/9/19 12:42 PM, Chris Mason wrote:

>> Looking at the patch, why is fsync special?  Seems like you could add
>> this ordering bit to any write?
>
> It's really not, the exact same technique could be used on any type of
> command to imply ordering. My initial idea was to have an explicit
> barrier/ordering command, but I didn't think that separating it from 
> an
> actual command would be needed/useful.

Might want to order discards?  Commit a transaction to free some blocks, 
discard those blocks?

>
>> While you're here, do you want to add a way to FUA/cache flush?
>> Basically the rest of what user land would need to make their own
>> write-back-cache-safe implementation.
>
> FUA would be a WRITEV/WRITE_FIXED flag, that should be trivially 
> doable.
>
> In terms of cache flush, that's very heavy handed (and storage
> oriented).  What applications would want/need to do an explicit whole
> device flush?

Basically if someone is writing a userland filesystem they'd want to 
cache flush for the same reasons the kernel filesystems do.


-chris