HP StoreEver Ultrium Tape Drives - Manual (Page 12)

Usage details
Format version compatibility
Earlier versions of the HP LTFS application supported the previous LTFS format specification; version
2.1.0 onwards supports the updated LTFS format specification. LTFS volumes created with application
v2.1.0 or later will not be readable in older versions of the LTFS application, as shown in the
following table:
Table 1 LTFS compatibility
Cartridge written by v2.1.0 or later
Cartridge written by LTFS application
v1.1.0 or earlier
LTFS application
Cannot be mounted Can be read and written v1.1.0 or earlier
Can be read and written Can be read; will automatically be
converted to the new format if written v2.1.0 or later
HP recommends that all instances of HP HP LTFS should be updated to the latest version of SOS
to avoid these compatibility issues.
If, for some reason, it is important to maintain cartridges readable by previous versions of the HP
LTFS application, care should be taken to avoid writing to those cartridges with the new version
of the application. This may be accomplished by mounting the volume as read-only, or by using
the cartridge write-protect tab to prevent cartridge updates. However, this should be regarded as
a temporary measure until all the application instances can be updated to v2.1.0 or later.
Use of sync_type options
Previous versions of HP LTFS supported a single mechanism for updating the tape index, in which
the index was written to tape only when the volume was unmounted. In normal operation this is
sufficient since the LTFS usage model requires the volume to be unmounted after use, so there is
always a valid index. However, if power is removed from the drive without unmounting, for example
due to an unplanned power outage or accidental unplugging, the volume will be left in an
inconsistent state and all files added since the last dismount operation will be inaccessible. The
ltfsck
utility may be able to recover the file data to the _ltfs_lostandfound directory but
the metadata (such as filename, access dates) will be lost.
To address this exposure, HP LTFS offers two additional mechanisms for updating the tape index.
The first is to write a copy of the index to tape periodically while writing data; this is the new
default behavior and the index is written out every five minutes. The time interval can be set with
a resolution of one minute. This method of index update reduces the window of exposure to power
loss events, since there will always be an index on tape no more than five minutes old (if using the
default interval).
The second additional mechanism provided in HP LTFS is to update the index every time a file is
closed after writing. This further reduces the exposure window to a single file, or (if several processes
are using the volume simultaneously) to the number of files being written concurrently. However,
this does add some overhead to LTFS, both in terms of the tape capacity used for each index, and
also in terms of performance since the writing of the index will take bandwidth away from writing
data. These overhead effects are more severe with smaller file sizes.
In most cases the default HP LTFS behavior offers the best solution, but the new features allow the
behavior to be tailored to a particular situation. The following table summarizes the options now
available for choosing when the index is updated.
12 Using HP LTFS