Re: [PATCH] FAT: use blkdev_issue_flush() instead of congestion_wait()

From: OGAWA Hirofumi
Date: Mon Nov 22 2021 - 00:30:00 EST

"NeilBrown" <neilb@xxxxxxx> writes:

> On Sun, 21 Nov 2021, OGAWA Hirofumi wrote:
>> "NeilBrown" <neilb@xxxxxxx> 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).

# cp -a /foo/source /mnt/fatfs

# umount <don't too slow>
<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@xxxxxxxxxxxxxxxxxx>