> On Mon, 26 Jun 2023, Mikulas Patocka wrote:
>
> > The BLKFLSBUF ioctl doesn't send the flush bio to the block device, thus
> > flushed data may be lurking in the disk cache and they may not be really
> > flushed to the stable storage.
> >
> > This patch adds the call to blkdev_issue_flush to blkdev_flushbuf.
>
> Umm, why? This is an ioctl no one should be using, and we certainly
> should not add new functionality to it. Can you explain what you're
> trying to do here?
Marc Smith reported a bug where he wrote to the dm-writecache target using
O_DIRECT, then reset the machine without proper shutdown and the freshly
written data were lost. It turned out that he didn't use the fsync or
fdatasync syscall (and dm-writecache makes its metadata persistent on a
FLUSH bio).
When I was analyzing this issue, it turned out that there is no easy way
how to send the FLUSH bio to a block device from a command line.
The sync command synchronizes only filesystems, it doesn't flush cache for
unmounted block devices (do you think that it should flush block devices
too?).
The "blockdev --flushbufs" command also doesn't send the FLUSH bio, but I
would expect it to send it. Without sending the FLUSH bio, "blockdev
--flushbufs" doesn't really guarantee anything.
Mikulas
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