linux-kernel - Re: [f2fs-dev] [PATCH v2] f2fs: introduce fragment allocation mode mount option

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Date:   Sat, 28 Aug 2021 08:43:11 +0800
From:   Chao Yu <chao@...nel.org>
To:     Daeho Jeong <daeho43@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: introduce fragment allocation mode
 mount option

On 2021/8/27 23:22, Daeho Jeong wrote:
>> I'd like to add a fixed chunk/hole fragmentation mode in addition, then
>> userspace can control the fragmented chunk/hole with fixed size.
>>
>> How do you think of renaming "fragment:block" to "fragment:rand_block", and
>> then I can add "fragment:fixed_block" option and its logic in addition?
>>
> 
> The reason I added the randomness on these values is the segment
> selection in SSR mode.
> If all the segments have the same free block counts, f2fs will
> allocate a new segment sequentially in SSR.

I'm fine with this, since test program can customize different fragment
degree on segments by setting different chunk/hole size.

> This was what I didn't want. Plus, in the real world, the size of hole
> and chunk will be different in different segments.
> 
> But, if you think we need this "fragment:fixed_block" mode, I am happy
> to have it. :)

Thanks a lot. :)

As you said it needs to be aligned to real world fragmentation, I notice that:
with this way, we can't simulate similar fragment in FTL, due to its page
mapping architecture, all fragmented data/node in filesystem will be written
sequentially into its blocks.

In order to simulate fragment in FTL, we need to:
- write data (chunk size) with dummy pages (hole size) to devices
- issue discards on those holes

I guess fragmenting device (erase blocks) at the same time wouldn't be
original intention of this patch, right?

Thanks,

> 
>> Do we need to consider multiple thread scenario? in such case,
>> .fragment_remained_chunk may update randomly.
>>
>> In addition, multiple log headers share one .fragment_remained_chunk,
>> it may cause unexpected result, it means there may be continuous holes
>> or chunks in locality due to swithing between different log headers.
>>
>> Thanks,
>>
> 
> Oh, I overlooked that point. I am going to add the variable for each
> segment as you said before.
> 
> Thanks,
>