When is write-behind data valid?
From: Jamie (HeyJames_ca_at_hotmail.com)
Date: 12/30/03
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Date: 30 Dec 2003 13:01:15 -0800
There's something I need to know about write-behind disk caches once
and for all. I always assumed that the write-behind part and the
read-ahead part of a cache worked together, i.e. if something writes
some data to disk, that data will be available for reading as soon as
it is in the write-behind cache. From what I read about fflush and
sync, they make it seem like this won't just be a problem when the
power suddenly goes out. They talk about it like the read-ahead buffer
will only have stuff that's been physically written on the disk. Is
this true?
What's worse is that the man pages seem to say that you can *never*
know if the data in a write-behind cache is valid until the file
system is umounted. The man pages for fflush say it doesn't actually
flush the cache, but something else. The notes in fsync say that it
doesn't guarantee that things in the write-behind cache get written
either. Only the man pages for sync seem to say that it really does
sync.
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