Never Best wrote:
Sorry I couldn't find this anywhere yet. For deduping it is best to have the
lookup table in RAM, but I wasn't too sure how much RAM is suggested?
::Assuming 128KB Block Sizes, and 100% unique data:
1TB*1024*1024*1024/128 = 8388608 Blocks
::Each Block needs 8 byte pointer?
8388608*8 = 67108864 bytes
::Ram suggest per TB
67108864/1024/1024 = 64MB
So if I understand correctly we should have a min of 64MB RAM per TB for
deduping? *hopes my math wasn't way off*, or is there significant extra
overhead stored per block for the lookup table? For example is there some kind
of redundancy on the lookup table (relation to RAM space requirments) to
counter corruption?
I read some articles and they all mention that there is significant performance
loss if the table isn't in RAM, but none really mentioned how much RAM one
should have per TB of duping.
Thanks, hope someone can confirm *or give me the real numbers* for me. I know
blocksize is variable; I'm most interessted in the default zfs setup right now.
There were several detailed discussions about this over the past 6
months that should be in the archives. I believe most of the info came
from Richard Elling.
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