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Linear Tape File System (LTFS) Caveats

Views: 20716
Votes: 8
Posted: 11 May, 2011

TOLIS Group has compiled the following list of caveats for the Linear Tape File System (LTFS ) that we feel users of LTFS need to be made aware of before they begin use of LTFS.

Updated August 26, 2014

Disclaimer: While TOLIS Group provides the files and drivers necessary to enable LTFS support for your TOLIS Group LTO -5 or LTO-6 tape bundle, we do not provide support for these files. The files are simply provided as a courtesy to TOLIS Group customers and they are provided under the various licenses included with the LTFS support files and are provided as is with no warranty, implied or otherwise. Use of the LTFS technology is strictly at your risk as TOLIS Group does not claim any responsibility for the code or binary files provided in the packages nor the operation of the LTFS layer on your system.

  1. Open source means no specific vendor support. One of the most telling entries about LTFS and its status as "open Source" is in the README file included with both the source code and the compiled binaries.  "LTFS is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details."  Who do you hold responsible if it doesn't work?  Also, don't let the term "open source" lull you into a sense of it being  a good thing.  Ask yourself this question - "Do I, or someone in my organization, know enough about C/C++ and low level tape drive operation on my specific system to deal with the source code if something happens?"
  2. Disable System Sleep and Hibernation when using LTFS. Both OS X and Windows systems will corrupt any mounted LTFS volume if the system goes to sleep or hibernates.  While a few have reported limited recovery success with the deep recovery option of the ltfsck tool, most volumes are not recoverable.
  3. Do not use LTFS as a Live Filesystem.  LTFS is best for Nearline storage and as such, LTFS volumes should be primarily used for archival or data sharing.  While you can actively access files from an LTFS volume, we do not recommend that you do so.  Simultaneously reading a file with an application and then saving the changes back to the tape results in delayed I/O and the loss of the original file's size in capacity on the tape since replacing an existing file does not free the original file's space on the tape, simple saving of changes will result in very slow application operation and a reduction in the apparent capacity of the LTFS volume (see item 7, below).
  4. Use Finder / Explorer (and Finder replacements) sparingly.  Because the OS X Finder and Windows Explorer try to retrieve file information for folders and files as they are displayed, every time you use Finder to access and expand folders on a mounted LTFS volume, you'll be required to wait while the tape is accessed to retrieve the freshly exposed files and folders.  Also, while Finder / Explorer is seeking on the tape, all other Finder / Explorer operations will be held (you will see the spinning beach ball or busy wait cursor) until the LTFS operation completes.  This can impact other applications as well.
  5. Multiple Simultaneous Access is not recommended.  While Windows or OS X and the MacFUSE driver will not report an error if you try to simultaneously access multiple files from the tape, or perform read and write operations at the same time, the resulting tape partition changes and tape seeks will result in an extended wait for all operations to complete.
  6. Do not access the tape between writes If you are writing multiple segments of data.  If you are writing data from multiple sources in multiple write operations (i.e.: Copying the contents of multiple USB drives onto a single LTFS tape), it is faster to write each new segment of data to the LTFS volume with no further access of the tape between writes.  This allows the drive to keep the tape at the current end of data (EOD) position so that the next segment can start writing immediately.  If you access the LTFS volume between the write operations (i.e.: Open the volume in Finder), the drive will need to change to the Index partition and then seek back to the data being viewed.  When you then start the next write operation, you will need to wait while the tape is positioned back to EOD before the write can continue.
  7. Additional write access will run more slowly than the first write.  In addition to item 4 above, performing multiple write sequences - especially after a tape has been unmounted and remounted - will slow down progressively as more data is added.  This is caused by the actions required to update the index information and reposition the tape between the partitions and then move to EOD.
  8. Updating files does not release previously used space.  Since "overwriting" existing files on an LTFS volume does not free up the space used by the previous copy of the file, you will lose space on the volume equal to the size of the file that was replaced.  After many updates, this can result in a tape that has far less than the rated 1.5TB of space.  The only way to recover the space is to copy the current contents to another volume, reformat the LTFS volume and then copy the files to the freshly formatted volume.
  9. Do not use an LTFS volume to play back Music or Video files.  This is similar to the heading "Do not use an LTFS volume as a live filesystem."  Because the MacFUSE layer only caches a select amount of data, using an LTFS volume as a music storage solution will result in choppy and failed playback as the tape seeks the non-cached portion of the currently playing song or video.
  10. LTFS doesn't support spanning volumes. Unlike BRU products, if you are writing to an LTFS volume you must be aware of how much data you've written to the tape since LTFS, like disk, does not have a mechanism for prompting for a new tape.  If you attempt to write more data than the tape will hold, you will simply get a "no space left on device" error and the write will fail.  In the same situation, BRU PE will safely write what it can onto the current tape and then prompt for an additional tape and complete the operation on the new volume.  There are a few wrappers that manage this somewhat better, but they are not LTFS and are also not free.
  11. Do not depend on OS X or Windows system disk tools for information about an LTFS volume.  Because system tools like Finder's "Get Info...", "du", and "df" do not have logic for dealing with the compression on an LTO drive or the space lost to file rewrites on an LTFS volume, the values returned will be estimates that will become less accurate as you write more data to an LTFS volume or replace existing files with new versions.  While the available space numbers will be correct, if added to the used space in such situations, the resulting value will be less than the actual stated capacity for a tape.
  12. LTFS doesn't offer a verification mechanism. Unlike BRU formatted tapes, LTFS volumes can only be verified on the system where they were created and written and then only by performing a full file-by-file comparison against the original files on disk or by completely restoring the files to another disk volume.
  13. LTFS tapes can't be used for normal backup and archival operations.  Once a tape is formatted for use as an LTFS volume, it is not possible to use the tape with a normal "backup" or "archival" software package.  This means that tapes defined for LTFS use can only be used for LTFS purposes.  If you are only testing the LTFS waters, be sure that you're not depleting your normal backup media pool since you'll need to "unltfs" any tapes that you use in testing to use that tape with your normal backup application.
Concerns when using LTFS with a tape Library
  1. When unmounting LTFS Volumes when using automation, improper handling of the library can cause data loss on LTFS tapes in the library. When unmounting a library in an LTFS configuration, the umount command will return immediately. However, cached data will continue to be written to tape for a few minutes. If there are two tape drives, the cached writes may take up to 5 minutes total. Do not power off or reset the tape library for at least 5 minutes after the umount command has been issued. Otherwise, the tapes may become inconsistent and potential data loss may occur. For similar reasons, wait at least 5 minutes after umount has returned before re-mounting the library.
  2. The Library version of the LTFS tools uses a format definition that differs from the format definition used with the standalone version. The Library-specific version of the LTFS drivers and tools now requires that formatting a tape use the 8 character barcode label for the tape's serial number.  This is incompatible with the 6 character serial number support by the standalone LTFS driver and tools. This can prevent you from being able to format tapes using the library LTFS tools if your tapes either do not have barcodes, have barcodes that do not include the tape type identifier string (L5 or L6), or where the library is set to not report the tape type fields for loaded tapes.

    Tapes in the library written using the BRU format are not affected by these in any case.
Using BRU PE's Cataloging to Manage LTFS Media

Note: The items listed below are specific to the BRU PE LTFS integration/support that exists within the product do not apply when using LTFS outside of BRU PE.

  1. Use unique volume serial numbers.  When creating LTFS tapes (formatting) with BRU PE's LTFS Management Console, please verify that you are using distinct and unique volume serial numbers.  It is these serial numbers that allow BRU PE to distinguish between tapes that you import for cataloging.  If you import multiple tapes with the same volume serial number, the later imports will overwrite the earlier imported tape catalogs since this appears to BRU PE's import process as updating the catalog for a known, existing LTFS volume.
  2. Do not use the library-specific LTFS tools even if you are using a library. Only use the standalone tools for your drive type and manually load and unload the tapes using either the Tools panel in BRU PE, the library's control panel or web interface, or the libctl command line tool.
  3. Re-import LTFS tapes after new data is added. If you are using BRU Producer's Edition to manage you LTFS tape collection, you must update the BRU PE catalog Info when you add data to an existing tape.  If you are adding new data to a tape that was previously imported into BRU PE, the catalog must be updated by reimporting the tape.  Because BRU PE only knows about LTFS volumes through the state of their catalog info, files added to an existing tape will not be seen in that volume's catalog unless you reimport that volume.  The new import will replace the existing catalog.
 

Last Updated: August 26, 2014

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