On Tue, Jun 27, 2023 at 05:31:12PM +0200, Mikulas Patocka wrote: > 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). Which so far is expected. Even with O_DIRECT you need O_(D)SYNC or fsync/fdatasync to persist data. > 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. xfs_io -c fsync /dev/foo > 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. I wouldn't expect it. It's a really weird legacy thing that calls back up into the file system, but only if it sets s_bdev to this device. I don't think we should add new users of it that overload the semantics.
