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Apps That Rely On Ext3's Commit Interval May Lose Data In Ext4

pc486 Re:Exactly (830 comments)

Lots of small files isn't bad on its own. In fact, it's downright common. Ext4's design does consider this case and makes these operations efficient.

The problem with small files is data consistency. If the application requires a file hierarchy and associated buffers to be on disk before continuing, then a call to fsync() is required (even on ext3). Implicitly syncing on every small file will kill performance, so don't do that.

more than 5 years ago
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Apps That Rely On Ext3's Commit Interval May Lose Data In Ext4

pc486 Re:Bull (830 comments)

Ext3 doesn't write out immediately either. If the system crashes within the commit interval, you'll lose whatever data was written during that interval. That's only 5 seconds of data if you're lucky, much more data if you're unlucky. Ext4 simply made that commit interval and backend behavior different than what applications were expecting.

All modern fs drivers, including ext3 and NTFS, do not write immediately to disk. If they did then system performance would really slow down to almost unbearable speeds (only about 100 syncs/sec on standard consumer magnetic drives). And sometimes the sync call will not occur since some hardware fakes syncs (RAID controllers often do this).

POSIX doesn't define flushing behavior when writing and closing files. If your applications needs data to be in NV memory, use fsync. If it doesn't care, good. If it does care and it doesn't sync, it's a bad application and is flawed, plain and simple.

more than 5 years ago

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