[stgt] [PATCH] SSC: dont fsync() on each written block. Do one single fsync() when file is closed in FILEMARK

[ronnie sahlberg]

Tomo, list,

Please find attached a patch to SSC which changes when it performs fsync().
With this patch fsync will not be called for normal block writes but
only for the final "write a filemark" that is issued when a full
session writing to the tape has finished.

The patch does improve performance significantly, especially on
slow/low-end/dodgy hardware where fsync is expensive.


As for data safety, I think this is ok.
We dont guarantee that the data is all destaged to physical media
until WRITE-FILEMARK command has completed successfully.

Imho if the system crashes during the write session but before it has
completed (==WRITE-FILEMARK) then all bets are off ?


Maybe Mark and some of the other tape people can speak up if this is
valid or dodgy reasoning?
If so maybe I could add a attribute "buffered write = Yes|No" where
fsync() on every write vs fsync() one the backup is complete
can be controlled.

Performance improvement is significant on slow hw!


regards
ronnie sahlberg
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