Sean Barrett on Twitter: "@cmuratori @won3d It depends on what got recycled. If something high in the tree got recycled... again, see the link I gave for more details"

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Casey Muratori ‏@cmuratori 17 Oct 2013

@cmuratori Why wouldn't you store uberblocks _strided_ across the drive instead?
Casey Muratori ‏@cmuratori 17 Oct 2013

@cmuratori The ZFS people seem smart so I'm wondering if I'm just not getting why it's not a problem for the uberblocks to be contiguous.
Won Chun ‏@won3d 17 Oct 2013

@cmuratori I'm guessing uberblocks are replicated. And they are probably not striped because syncs across disks aren't transactional.
Sean Barrett ‏@nothings 17 Oct 2013

@won3d @cmuratori (2) from same link: only *1* uberblock is actually the current valid root, you don't care what's in the other 127 anyway
Casey Muratori ‏@cmuratori 17 Oct 2013

@nothings @won3d Yes but that's not relevant here. Since the 128 are stored together, if that sector/pagegoes they will all go.
Sean Barrett ‏@nothings 17 Oct 2013

@cmuratori @won3d But only one of them is the current root. The other 127 are stale roots, so you don't care if you lose them.
Casey Muratori ‏@cmuratori 17 Oct 2013

@nothings @won3d No, that is incorrect. They are all "valid roots" due to COW, so in case the latest one had a bad write, you are OK.
Sean Barrett ‏@nothings 17 Oct 2013

@cmuratori @won3d Obviously that doesn't work once free space gets recycled. But you're right: http://utcc.utoronto.ca/~cks/space/blog/solaris/ZFSMetadataRecovery …
Casey Muratori ‏@cmuratori 17 Oct 2013

@nothings @won3d It actually does work even when free space gets recycled, just not for the entire tree. So you still get _some_ protection.
Casey Muratori ‏@cmuratori 17 Oct 2013

@nothings @won3d You'll only lose the part that actually got recycled, rather than the whole drive which is what would happen otherwise.

‏@nothings

@cmuratori @won3d It depends on what got recycled. If something high in the tree got recycled... again, see the link I gave for more details

2:50 PM - 17 Oct 2013

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