On 3/4/13 5:20 PM, Mark Campbell wrote:

> Oh, and while I'm thinking of it, what are your thoughts on using ZFS' dedup 
> feature on a BackupPC pool?  I'm aware that a goodly amount of RAM would be 
> required for that feature.  But since BackupPC's dedup feature is file-based, 
> and ZFS' dedup feature is block-based, even more space could be saved; 
> particularly when you're backing up things like .pst files, where a large 
> majority of the file is the same, save a few bytes/KB/MB.  Such files are 
> flagged by BackupPC as different.

As long as you can keep the DDT in RAM, it would not slow down too much.
You should test that, though.

For about 1 TB of used disk space, I believe you would need something
like 3 GB of RAM, depending on your filesystem's average block size. You
can calculate the required RAM amount with "zdb -S trunk" which will
simulate dedup on your disk, and then you can multiply the total blocks
number by 320 to get the required RAM.

If the dedup tables (DDTs) spill over your amount of available RAM, it
will use the L2ARC if you have one or the disk if you don't have L2ARC
cache. Harddisk access for the DDTs would slow you down significantly.
If you can keep a pretty decent sized L2ARC on a very fast SSD it would
be less slow if your RAM is too small.

Also, like Tyler said, you should disable BackupPC's pooling to increase
your performance.

-- 
Best regards,
Lars Tobias

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