Most importantly a high performance filesystem needs to be able to sync the
data of one file independent of all the pending data for every other open
file. That is the whole problem with ext3 - it doesn't do that, so an fsync
under competing write load is very slow.
Ext4 fixes these problems, but either requires an fsync or inserts one to
make a rename replacement an atomic operation. That delay could be avoided
with some reasonable internal modifications (keeping the old inode around
until the new inode's data commits, and then undoing the rename if necessary
on journal recovery), but I am not aware of any filesystem that actually does
that. You have to call fsync to make your code portable anyway, but there
are a number of applications where that is too expensive.
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