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Subject: Re: [PATCH] bcache: add REQ_FUA to avoid data lost in writeback mode
From:   Coly Li <colyli@suse.de>
To:     Eric Wheeler <bcache@lists.ewheeler.net>
Cc:     kungf <wings.wyang@gmail.com>, kent.overstreet@gmail.com,
        linux-bcache@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20191202102409.3980-1-wings.wyang@gmail.com>
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Organization: SUSE Labs
Message-ID: <2d085823-57ac-dba9-6d2e-1cf01c949875@suse.de>
Date:   Wed, 4 Dec 2019 18:55:08 +0800
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On 2019/12/3 10:21 下午, Coly Li wrote:
> On 2019/12/3 3:34 上午, Eric Wheeler wrote:
>> On Mon, 2 Dec 2019, Coly Li wrote:
>>> On 2019/12/2 6:24 下午, kungf wrote:
>>>> data may lost when in the follow scene of writeback mode:
>>>> 1. client write data1 to bcache
>>>> 2. client fdatasync
>>>> 3. bcache flush cache set and backing device
>>>> if now data1 was not writed back to backing, it was only guaranteed safe in cache.
>>>> 4.then cache writeback data1 to backing with only REQ_OP_WRITE
>>>> So data1 was not guaranteed in non-volatile storage,  it may lost if  power interruption 
>>>>
>>>
>>> Hi,
>>>
>>> Do you encounter such problem in real work load ? With bcache journal, I
>>> don't see the possibility of data lost with your description.
>>>
>>> Correct me if I am wrong.
>>>
>>> Coly Li
>>
>> If this does become necessary, then we should have a sysfs or superblock 
>> flag to disable FUA for those with RAID BBUs.
> 
> Hi Eric,
> 
> I doubt it is necessary to add FUA tag for all writeback bios, it is
> unnecessary. If power failure happens after dirty data written to
> backing device and the bkey turns into clean, a following read request
> will go to cache device because the LBA can be indexed no matter it is
> dirty or clean. Unless the bkey is invalidated from the B+tree, read
> will always go to cache device firstly in writeback mode. If a power
> failure happens before the cached bkey turns from dirty to clean, just
> an extra writeback bio flushed from cache device to backing device with
> identical data. Comparing the FUA tag for all writeback bios (it will be
> really slow), the extra writeback IOs after a power failure is more
> acceptable to me.

Hi Eric,

I come to realize what the problem is. Yes there is potential
possibility.With FUA the writeback performance will be very low, it is
quite tricky....

Coly Li