From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH] ext4: hand over jobs handling discard commands on commit
 complete phase to kworkers
Date: Tue, 16 May 2017 04:53:06 -0700
Message-ID: <20170516115306.GA15058@infradead.org>
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To: Daeho Jeong <daeho.jeong@samsung.com>
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On Tue, May 16, 2017 at 04:37:42PM +0900, Daeho Jeong wrote:
> Now, when we mount ext4 filesystem with '-o discard' option, we have to
> issue all the discard commands for the blocks to be deallocated and
> wait for the completion of the commands on the commit complete phase.

No, you don't.  You can execute them asynchronously, similar to what
XFS already does.