linux-kernel - Re: [PATCH v3 7/7] zram: writeback throttle

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Date:   Wed, 28 Nov 2018 15:41:41 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Joey Pabalinas <joeypabalinas@...il.com>
Subject: Re: [PATCH v3 7/7] zram: writeback throttle

On Tue, 27 Nov 2018 14:54:29 +0900 Minchan Kim <minchan@...nel.org> wrote:

> On small memory system, there are lots of write IO so if we use
> flash device as swap, there would be serious flash wearout.
> To overcome the problem, system developers need to design write
> limitation strategy to guarantee flash health for entire product life.
> 
> This patch creates a new konb "writeback_limit" on zram. With that,
> if current writeback IO count(/sys/block/zramX/io_stat) excceds
> the limitation, zram stops further writeback until admin can reset
> the limit.

I'm not really understanding this.  Does this only refer to suspending
the idle page writeback feature?  Not all zram writeback, surely?

I don't think the documentation gives an administrator sufficient
information to effectively use the feature.  Some additional discussion
would help.  What sort of values should it be set to and why?

And what is the default setting?  And why?

And the limit isn't persistent across reboots which makes me wonder
whether the overall feature is particularly valuable?