On Apr 2, 2014, at 3:08 PM, Matt Elliott <melli...@ncsa.illinois.edu> wrote:

> Not true.  ZFS flushes also mark known states.  If the zfs stack issues a 
> flush and the system returns, it uses that as a guarantee that that data is 
> now on disk.

However, that guarantee is only needed to ensure that on-disk data is 
consistent even if the contents of the cache is lost, e.g. due to sudden power 
loss. A disk cache never just loses dirty data in normal operation.

> later writes will assume that the data was written and if the hard drive 
> later changes the write order (which some disks will do for performance) 
> things break.  You can have issues if any part of the disk chain lies about 
> the completion of flush commands.

What would break, in your opinion? Again, as long as you don’t somehow lose the 
contents of your cache, it really doesn’t matter at all what’s physically on 
the disk and what’s still in the cache.

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