Subject: Re: [LSF/MM TOPIC] More async operations for file systems - async
 discard?
To:     Dave Chinner <david@fromorbit.com>
Cc:     lsf-pc@lists.linux-foundation.org,
        linux-xfs <linux-xfs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>,
        linux-block@vger.kernel.org
References: <92ab41f7-35bc-0f56-056f-ed88526b8ea4@gmail.com>
 <20190217210948.GB14116@dastard>
From:   Ric Wheeler <ricwheeler@gmail.com>
Message-ID: <46540876-c222-0889-ddce-44815dcaad04@gmail.com>
Date:   Sun, 17 Feb 2019 18:42:59 -0500
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On 2/17/19 4:09 PM, Dave Chinner wrote:
> On Sun, Feb 17, 2019 at 03:36:10PM -0500, Ric Wheeler wrote:
>> One proposal for btrfs was that we should look at getting discard
>> out of the synchronous path in order to minimize the slowdown
>> associated with enabling discard at mount time. Seems like an
>> obvious win for "hint" like operations like discard.
> We already have support for that. blkdev_issue_discard() is
> synchornous, yes, but __blkdev_issue_discard() will only build the
> discard bio chain - it is up to the caller to submit and wait for it.
>
> Some callers (XFS, dm-thinp, nvmet, etc) use a bio completion to
> handle the discard IO completion, hence allowing async dispatch and
> processing of the discard chain without blocking the caller. Others
> (like ext4) simply call submit_bio_wait() to do wait synchronously
> on completion of the discard bio chain.
>
>> I do wonder where we stand now with the cost of the various discard
>> commands - how painful is it for modern SSD's?
> AIUI, it still depends on the SSD implementation, unfortunately.

I think the variability makes life really miserable for layers above it.

Might be worth constructing some tooling that we can use to validate or shame 
vendors over - testing things like a full device discard, discard of fs block 
size and big chunks, discard against already discarded, etc.

Regards,

Ric