linux-kernel - Re: [PATCH] FAT: use blkdev_issue_flush() instead of congestion

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Date:   Mon, 22 Nov 2021 14:29:53 +0900
From:   OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:     "NeilBrown" <neilb@...e.de>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH] FAT: use blkdev_issue_flush() instead of congestion_wait()

"NeilBrown" <neilb@...e.de> writes:

> On Sun, 21 Nov 2021, OGAWA Hirofumi wrote:
>> "NeilBrown" <neilb@...e.de> writes:
>> 
>> > congestion_wait() in this context is just a sleep - block devices do not
>> > in general support congestion signalling any more.
>> >
>> > The goal here is to wait for any recently written data to get to
>> > storage.  This can be achieved using blkdev_issue_flush().
>> 
>> Purpose of flush option should be for making umount faster, not data
>> integrity. (but current flush implement is strange at several places, IMO)
>
> I don't think that is true.  I believe the purpose of the flush option
> is to write out data as soon as a file is closed, so that if the media
> is removed without first unmounting, the data is more likely to be safe.
> That is why the commit which introduce it:
>  Commit ae78bf9c4f5f ("[PATCH] add -o flush for fat")
> particularly mentions "removable media".

Right. This was to make the removable device usage better (but sync
option is too slow).

e.g.
    # cp -a /foo/source /mnt/fatfs

    # umount <don't too slow>
    or
    <do other thing, and forget umount>

>> So, I don't think the issue_flush is not proper for it (flush is very
>> slow on some usb thumb), and rather I think it is better off to just
>> remove the congestion_wait().
>
> We already call blkdev_issue_flush() on fsync.  With my patch, a simple
> close() effective becomes an fsync() and a close().  I think that is
> completely consistent with the purpose of "-o flush".

It makes much slower above "cp -a" part. So I think it is overkill.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>