Hammer deduplication needs for RAM size

Tomas Bodzar tomas.bodzar at gmail.com
Sat Apr 23 10:06:23 PDT 2011

On Fri, Apr 22, 2011 at 10:12 PM, Matthew Dillon
<dillon at apollo.backplane.com> wrote:
> :Hi all,
> :
> :can someone compare/describe need of RAM size by deduplication in
> :Hammer? There's something interesting about deduplication in ZFS
> :http://openindiana.org/pipermail/openindiana-discuss/2011-April/003574.html
> :
> :Thx
>    The ram is basically needed to store matching CRCs.  The on-line dedup
>    uses a limited fixed-sized hash table to remember CRCs, designed to
>    match recently read data with future written data (e.g. 'cp').
>    The off-line dedup (when you run 'hammer dedup ...' or
>    'hammer dedup-simulate ...' will keep track of ALL data CRCs when
>    it scans the filesystem B-Tree.  It will happily use lots of swap
>    space if it comes down to it, which is probably a bug.  But that's
>    how it works now.
>    Actual file data is not persistently cached in memory.  It is read only
>    when the dedup locates a potential match and sticks around in a limited
>    cache before getting thrown away, and will be re-read as needed.

Their discussion continues and they talk about rule 1 - 3GB of RAM per
1TB of data. Regarding this
http://blogs.sun.com/roch/entry/dedup_performance_considerations1 it
looks like those data are persistent as cache in memory. So is this a
reason for higher RAM usage with ZFS and dedup when comparing with

>                                        -Matt
>                                        Matthew Dillon
>                                        <dillon at backplane.com>

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