> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Jim Klimov
> 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). 

If they were scattered around the hypothetical dedicated DDT SSD, I would say, 
no problem.  But in reality, they're scattered in your main pool.  DDT writes 
don't get coalesced.  Is this simply because they're sync writes?  Or is it 
because they're metadata, which is even lower level than sync writes?  I know, 
for example, that you can disable ZIL on your pool, but still the system is 
going to flush the buffer after certain operations, such as writing the 
uberblock.  I have not seen the code that flushes the buffer after DDT writes, 
but I have seen the performance evidence.
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