linux-kernel - Re: [PATCH] bcache: add REQ_FUA to avoid data lost in writeback mode

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Date:   Mon, 2 Dec 2019 19:34:12 +0000 (UTC)
From:   Eric Wheeler <bcache@...ts.ewheeler.net>
To:     Coly Li <colyli@...e.de>
cc:     kungf <wings.wyang@...il.com>, kent.overstreet@...il.com,
        linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: add REQ_FUA to avoid data lost in writeback
 mode

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.

--
Eric Wheeler




> > Signed-off-by: kungf <wings.wyang@...il.com>
> > ---
> >  drivers/md/bcache/writeback.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> > index 4a40f9eadeaf..e5cecb60569e 100644
> > --- a/drivers/md/bcache/writeback.c
> > +++ b/drivers/md/bcache/writeback.c
> > @@ -357,7 +357,7 @@ static void write_dirty(struct closure *cl)
> >  	 */
> >  	if (KEY_DIRTY(&w->key)) {
> >  		dirty_init(w);
> > -		bio_set_op_attrs(&io->bio, REQ_OP_WRITE, 0);
> > +		bio_set_op_attrs(&io->bio, REQ_OP_WRITE | REQ_FUA, 0);
> >  		io->bio.bi_iter.bi_sector = KEY_START(&w->key);
> >  		bio_set_dev(&io->bio, io->dc->bdev);
> >  		io->bio.bi_end_io	= dirty_endio;
> > 
> 
>