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Re: [zfs-discuss] Dedup RAM requirements, vs. L2ARC?

Subject: Re: [zfs-discuss] Dedup RAM requirements, vs. L2ARC?
From: Neil Perrin
Date: Fri, 02 Jul 2010 07:30:15 -0600
On 07/02/10 00:57, Erik Trimble wrote:
On 7/1/2010 10:17 PM, Neil Perrin wrote:
On 07/01/10 22:33, Erik Trimble wrote:
On 7/1/2010 9:23 PM, Geoff Nordli wrote:
Hi Erik.

Are you saying the DDT will automatically look to be stored in an L2ARC device if one exists in the pool, instead of using ARC?

Or is there some sort of memory pressure point where the DDT gets moved from ARC to L2ARC?



Good question, and I don't know. My educated guess is the latter (initially stored in ARC, then moved to L2ARC as size increases).


The L2ARC just holds blocks that have been evicted from the ARC due
to memory pressure. The DDT is no different than any other object
(e.g. file). So when looking for a block ZFS checks first in the ARC then
the L2ARC and if neither succeeds reads from the main pool.

- Anyone.

That's what I assumed. One further thought, though. Is the DDT is treated as a single entity - so it's *all* either in the ARC or in the L2ARC? Or does it move one entry at a time into the L2ARC as it fills the ARC?

It's not treated as a single entity but at a block at a time.

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