Hi, Yes, this is a bug, thanks for looking into this. O_DIRECT was dropped because it was slow, causing some dump/restores to take minutes. There should be no problem putting it back in. Whether O_DIRECT is used or normal buffered IO shouldn't make a difference with normal offline dumping. However, you're dumping metadata snapshots from a live system, and somehow we're getting out of sync with the buffered io, and what's really on disk. I'd hoped we'd avoid this, the kernel is *not* changing the metadata snapshot after all. But I guess blocks are being held in the page cache longer than I expected. For now I'll change it so it uses O_DIRECT if the metadata_snap flag is given. Longer term I need to improve my user land cache a bit so it preemptively flushes dirty data when we get low on space. This will make O_DIRECT perform well. Thanks again, - Joe On Fri, Oct 04, 2013 at 03:01:14PM +0800, Teng-Feng Yang wrote: > Hi folks, > > I have been working on an incremental backup tools of thin volumes > created by dm-thin for a couple of months. > This tool relies on the data mappings dumped by thin_dump from the > thin-provisioing-tools. > As I update thin_dump from version 0.1.5 to 0.2.7, it cannot work > correctly as the earlier 0.1.5 version. > > The following things have been observed: > 1. thin_dump-0.2.7 cannot take user-specified "metadata_snap" as > v0.1.5. It will always return "bad checksum in superblock". > 2. If "metadata_snap" is not given by users, thin_dump-0.2.7 only > dumps the out-dated data mappings which can be recognized by observing > the wrong "transaction id". > > Since it looked like thin_dump failed to get the most up-to-date > metadata blocks in version 0.2.7, I dig a little deeper into the > source code and find something that might cause the problem. > The open flag used on metadata device has been changed from "O_DIRECT > | O_SYNC" to "0" in commit 4deb1751a650010ce9c15910a064be1348dec8ba . > After I changed this back to "O_DIRECT | O_SYNC" in version 0.2.7, it > works as expected once again. > > So here is my question. > Can I change this flag back to "O_DIRECT | O_SYNC" in any newer version? > I am worry about that I might accidentally mess something up by > changing this flag. > Also, what's the purpose to get rid of the "O_DIRECT" and "O_SYNC" > flags when we migrate to version 0.2.x? > > Any help would be highly appreciated. > > Best Regards, > Dennis -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
