2012-08-01 23:40, opensolarisisdeadlongliveopensolaris пишет:

Agreed, ARC/L2ARC help in finding the DDT, but whenever you've got a snapshot 
destroy (happens every 15 minutes) you've got a lot of entries you need to 
write.  Those are all scattered about the pool...  Even if you can find them 
fast, it's still a bear.

No, these entries you need to update are scattered around your
SSD (be it ARC or a hypothetical SSD-based copy of metadata
which I also "campaigned" for some time ago). We agreed (or
assumed) that with SSDs in place you can find the DDT entries
to update relatively fast now. The values are changed in RAM
and flushed to disk as part of an upcoming TXG commit, likely
in a limited number of disk head strokes (lots to coalesce),
and the way I see it - the updated copy remains in the ARC
instead of the obsolete DDT entry, and can make it into L2ARC
sometime in the future, as well.

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