linux-ext4 - Re: [PATCH v4] ext4: change sequential discard handling on commit complete phase into parallel manner

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Date:   Thu, 22 Jun 2017 23:58:13 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Daeho Jeong <daeho.jeong@...sung.com>
Cc:     jack@...e.com, hch@...radead.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v4] ext4: change sequential discard handling on commit
 complete phase into parallel manner

On Wed, Jun 14, 2017 at 11:34:42AM +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.
> Because this procedure might involve a lot of sequential combinations of
> issuing discard commands and waiting for that, the delay of this
> procedure might be too much long, even to 17.0s in our test,
> and it results in long commit delay and fsync() performance degradation.
> 
> To reduce this kind of delay, instead of adding callback for each
> extent and handling all of them in a sequential manner on commit phase,
> we instead add a separate list of extents to free to the superblock and
> then process this list at once after transaction commits so that
> we can issue all the discard commands in a parallel manner like XFS
> filesystem.
> 
> Finally, we could enhance the discard command handling performance.
> The result was such that 17.0s delay of a single commit in the worst
> case has been enhanced to 4.8s.
> 
> Signed-off-by: Daeho Jeong <daeho.jeong@...sung.com>
> Tested-by: Hobin Woo <hobin.woo@...sung.com>
> Tested-by: Kitae Lee <kitae87.lee@...sung.com>
> Reviewed-by: Jan Kara <jack@...e.cz>

Thanks, applied.  Since the one-line summary was longer than 80
characters, I shortened it to:

    ext4: send parallel discards on commit completions

    	       			    	   - Ted