I try to fully understand this entry:
"Many drives use a write back cache in order to speed up the performance
of writes. However, there are conditions such as power failure when the
write cache memory is never flushed to the actual disk. This causes
problems for XFS and journaled filesystems in general because they rely
on knowing when a write has completed to the disk. They need to know that
the log information has made it to disk before allowing metadata to go to
disk. When the metadata makes it to disk then the tail of the log can
move. So if the writes never make it to the physical disk, then the
ordering is violated and the log and metadata can be lost, resulting in
I have problems with: "When the metadata makes it to disk then the tail of
the log can move". What does that mean exactly?
What I imagine is this: XFS write transaction to its log and the log
grows. When writing the meta data changes of a complete transaction XFS
removes it from the log. Now when the metadata changes of a transaction
has been written completely but the transaction itself has not, it may
happen that a transaction is removed from the on disk log before it has
been written. But even when this does not happen there are metadata
changes on disk that the log doesn't know about.
So there are two situations where unordered writes can make a journalling
1) Metadata make it to disk before the transaction that belongs to them =>
There are metadata changes that XFS doesn't know about.
2) A transaction might be deleted from the log before it has been written
=> This leads to a corrupted log.
Is that correct and complete? Please give feedback.
You may use any of my text here to update / clarify the FAQ ;-)
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
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