Re: ext4 filesystem bad extent error review

[Eric Sandeen <sandeen@redhat.com>]

On 1/3/14, 9:48 AM, Theodore Ts'o wrote:
> On Fri, Jan 03, 2014 at 11:16:02AM +0800, Huang Weller (CM/ESW12-CN) wrote:
>>
>> It sounds like the barrier test. We wrote such kind test tool
>> before, the test program used ioctl(fd, BLKFLSBUF, 0) to set a
>> barrier before next write operation.  Do you think this ioctl is
>> enough ? Because I saw the ext4 use it. I will do the test with that
>> tool and then let you know the result.
> 
> The BLKFLSBUF ioctl does __not__ send a CACHE FLUSH command to the
> hardware device.  It forces all of the dirty buffers in memory to the
> storage device, and then it invalidates all the buffer cache, but it
> does not send a CACHE FLUSH command to the hardware.  Hence, the
> hardware is free to write it to its on-disk cache, and not necessarily
> guarantee that the data is written to stable store.  (For an example
> use case of BLKFLSBUF, we use it in e2fsck to drop the buffer cache
> for benchmarking purposes.)

Are you sure?  for a bdev w/ ext4 on it:

BLKFLSBUF
	fsync_bdev
		sync_filesystem
			sync_fs
				ext4_sync_fs
					blkdev_issue_flush


-Eric