Hi folks,
I've been looking at implementing the lazytime mount option for XFS,
and I'm struggling to work out what it is supposed to mean.
AFAICT, on ext4, lazytime means that pure timestamp updates are not
journalled and they are only ever written back when the inode is
otherwise dirtied and written, or they are timestamp dirty for 24
hours which triggers writeback.
This poses a couple of problems for XFS:
1. we log every timestamp change, so there is no mechanism
for delayed/deferred update.
2. we track dirty metadata in the journal, not via the VFS
dirty inode lists, so all the infrastructure written for
ext4 to do periodic flushing is useless to us.
These are solvable problems, but what I'm not sure about is exactly
what the intended semantics of lazytime durability are. That is,
exactly what guaranteed are we giving userspace about timestamp
updates when lazytime is used? The guarantees we have to give will
greatly influence the XFS implementation, so I really need to nail
down what we are expected to provide userspace. Can we:
a) just ignore all durability concerns?
b) if not, do we only need to care about the 24 hour
writeback and unmount?
c) if not, are fsync/sync/syncfs/freeze/unmount supposed
to provide durability of all metadata changes?
d) do we have to care about ordering - if we fsync one inode
with 1 hour old timestamps, do we also need to guarantee
that all the inodes with older dirty timestamps also get
made durable?
I really want to completely ignore all ordering and forced
durability requirements for lazytime (i.e. implement only periodic,
optimistic and freeze/unmount writeback), but I haven't found any
documentation of what durability lazytime is supposed to provide and
can really only guess what was intended from the ext4
implementation....
Clarity would be appreciated.
Cheers,
Dave.
--
Dave Chinner
da...@fromorbit.com
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