Hi Graham,
I have used ZFS with CoreStorage for a few months, I'm not aware of dataloss.
I didn't use the words data loss, because I wasn't referring to data loss. I guess the more accurate wording is data location.
Let me re-ask the question this way:
How can you guarantee you're backing up files using ZFS' snapshot feature, when the files may not physically be located on the ZFS filesystem? (i.e. because they're on the SSD due to recent activity)
Snapshots cannot extend to the SSD for obvious reasons, so Fusion actually breaks ZFS's snapshot/backup feature. This point needs to made very clear to those not quite knowledgeable enough about this, but are wise enough to use ZFS.
Wherever I use ZEVO with CoreStorage, I scrub regularly
mmm, this won't help you if there is data corruption occurring on the SSD. You'll simply be scrubbing already corrupt data.
Take this scenario:
Files get copied from the ZFS spindle to HFS SSD, therefore you don't 100% know that the SSD has written the data correctly because there's no checksum feature to verify the write transaction. The OS is just taking HFS's word that it has.
So, some time parses, the user performs quite a few reads & writes (i.e. saving/updating the files) and is loving the speed increase. And let's just say that the last write happened to incur an issue such as a power spike, brown-out, defective ssd cell, whatever.... We now have a situation where file(s) are not what they should be.
Some more time passes and the user has since moved onto something else and doesn't use the now corrupted file(s) for some time.
So Fusion kicks in and decides to copy the file(s) back to the ZFS spindle. This is where the corrupted file(s) are given to ZFS. ZFS is only going to write the already corrupted file(s) and "transfer" the existing problem onto a ZFS filesystem. ZFS can't help in this situation, in fact, it can now only guarantee that the corrupted file(s) don't become any more corrupt. Scrubbing at this point cannot undo the pre-existing problem or help.
Now this may be very well be making a mountain out of a mole hill, but it demonstrates my point more clearly that moving data away from a checksummed filesystem and then back again can introduce this issue. Hence I won't be using Fusion, I value my data too much. We already have to deal will errors in memory, hbas and nics, and software drivers, so using ZFS is not always going to help, but it's a big help.
This is why I have had my personal data on a ZFS filesystem for years now. I will never move these files away from a ZFS filesystem, especially when our children will be dealing with petabyte drives!! I'm wanting to protect our photos/videos and
memories for them just as much for me right now.
I don't want to start up a big "hoo har" about this, but more importantly, I think the main thing to know is:
Having Apple's Fusion move data away from a checksummed filesystem and then back again can introduce uncorrectable errors that ZFS cannot fix.
Using Apple's Fusion with a ZFS filesystem cannot guarantee that ZFS's snapshot feature will backup all your data.These are indisputable facts.