On 12/5/2014 8:11 AM, Daniel Pocock wrote: > On 05/12/14 13:57, Josh Fisher wrote: >> On 12/5/2014 1:03 AM, Daniel Pocock wrote: >>> On 05/12/14 00:43, Cejka Rudolf wrote: >>>> Daniel Pocock wrote (2014/12/04): >>>>> On 04/12/14 18:35, Kern Sibbald wrote: >>>>>> On 12/03/2014 08:49 PM, Daniel Pocock wrote: >>>>>>> Does Bacula checksum content on the spool disk before sending it to >>>>>>> tape? >>>>>>> >>>>>>> To be more explicit, if there is a single bit error on the spool disk, >>>>>>> will it be noticed before going onto tape or would it only be noticed in >>>>>>> future when a file is taken off the tape? >>>>>> Unless you are running ZFS for the spool disk, the error will only be >>>>>> noticed when the data is read from the tape. >>>>>> >>>>> In that case, it sounds like a good idea to use ZFS or Btrfs with >>>>> checksums enabled >>>> Hard drives use error correction/detection codes, so single bit error >>>> without any error indication is unlikely. Especially in case of spool >>>> disks, where datas are read shortly after write. >>>> >>> Unfortunately, that is completely untrue. Disks and IO subsystems do >>> not provide any guarantees that they will return the exact data that was >>> written. That is why modern filesystems have checksums. >>> >>> The actual corruption of data often occurs in the writing phase, so >>> whether you read back the sector in 5 minutes or 5 weeks, it will always >>> come back with some bit changed. I've seen this more than once >>> unfortunately. >> A write error is not detected without a read. There is no way to know, >> even at the hardware level, whether a particular area of the disk has >> the correct magnetization / charge without a subsequent read. Then there >> are RAM buffers and controllers in between the FS and the disk platter / >> MLC cell. When the FS detects a checksum error, it really has no way to >> know whether it was due to an incorrect area of disk or an incorrect bit >> of RAM, but it knows that it didn't read back what should have been >> written. Without the hardware error detection, the FS may detect false >> positives, while without the FS checksum there is no way to detect false >> negatives. Both hardware level and FS level error detection are >> required. That is why I think the ZFS claim of "not needing any special >> hardware" is a bit misleading, or at least depends on the definition of >> "special hardware". > > Should this go in the bug tracker then? A feature request for Bacula to > assume the spool disk filesystem may not be using checksums and > therefore Bacula should checksum content on the spool disk itself when > handing it off to tape?
Well, that would be a feature request, rather than a bug, but I don't think that is needed. The only way to be sure a tape is correct is to read it. Bacula already has verify jobs that read the tape and check it against the client's file checksums. ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=164703151&iu=/4140/ostg.clktrk _______________________________________________ Bacula-users mailing list Bacula-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bacula-users