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Where does it say that not even sync, much less fsync, is adequate for ensuring your data is written to disk and reliably retrievable?

Sorry - I read that thread a long time ago and I summarized my recollection of a lot of reading stimulated by it. In comment #56 Theodore says, of his recommended best practice for syncing from the application: It is not fool-proof, but then again ext3 was never fool-proof, either. But he doesn't say much about what the remaining risks are.

A point that was well made elsewhere (e.g. http://www.hitachigst.com/hdd/technolo/writecache/writecache.htm (see the first paragraph under "What is write caching?") and http://old.nabble.com/ext4-finally-doing-the-right-thing-td27186399.html) is that not only the operating/file system buffers data and potentially reorders operations - many disk drives have intelligent controllers that also buffer data and reorder writes, and the disk controllers don't necessarily write their data when you issue a sync at the operating/file system level.

Risk of data loss or corruption can be reduced by calling the sync functions from the application at critical points - I don't mean to discourage doing so - it can be good practice. But don't get carried away or system performance may be adversely impacted.

In addition, some file system operations create more risk than others, and consideration should be given to how the file system is used, as well as when it is synchronized to disk.

Finally, for a good balance of reliability and performance, there are many configuration settings that should be considered, in the operating system, file system, RAID and virtual disk system (be they encrypting, compressing or whatever) and in the disk drives and interfaces.


In reply to Re^3: fsyncing directories by ig
in thread fsyncing directories by betterworld

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