linux-kernel - Re: [PATCH v2] dm: optimize flushes

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Date: Thu, 23 May 2024 14:02:29 -0400
From: Mike Snitzer <snitzer@...nel.org>
To: Mikulas Patocka <mpatocka@...hat.com>
Cc: Benjamin Marzinski <bmarzins@...hat.com>,
	Yang Yang <yang.yang@...o.com>, Alasdair Kergon <agk@...hat.com>,
	dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] dm: optimize flushes

On Thu, May 23, 2024 at 07:46:25PM +0200, Mikulas Patocka wrote:
> Hi
> 
> Here I'm resending the patch, with more comments and explanations added.
> 
> Mikulas
> 
> 
> From: Mikulas Patocka <mpatocka@...hat.com>
> 
> Device mapper sends flush bios to all the targets and the targets send it
> to the underlying device. That may be inefficient, for example if a table
> contains 10 linear targets pointing to the same physical device, then
> device mapper would send 10 flush bios to that device - despite the fact
> that only one bio would be sufficient.
> 
> This commit optimizes the flush behavior. It introduces a per-target
> variable flush_bypasses_map - it is set when the target supports flush
> optimization - currently, the dm-linear and dm-stripe targets support it.
> When all the targets in a table have flush_bypasses_map,
> flush_bypasses_map on the table is set. __send_empty_flush tests if the
> table has flush_bypasses_map - and if it has, no flush bios are sent to
> the targets via the "map" method and the list dm_table->devices is
> iterated and the flush bios are sent to each member of the list.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> Suggested-by: Yang Yang <yang.yang@...o.com>

Nicely done, please feel free to stage for 6.11 (create a new
'dm-6.11' branch starting from 'dm-6.10' -- we'll need to rebase
dm-6.11 to 6.10-rc2 or so but at least we'll get this commit in the
pipeline, push to 'for-next').

Reviewed-by: Mike Snitzer <snitzer@...nel.org>