> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of Jim Klimov > > 2012-08-01 22:07, opensolarisisdeadlongliveopensolaris пишет: > > L2ARC is a read cache. Hence the "R" and "C" in "L2ARC." > > "R" is replacement, but what the hell ;) > > > This means two major things: > > #1 Writes don't benefit, > > and > > #2 There's no way to load the whole DDT into the cache anyway. So you're > guaranteed to have performance degradation with the dedup. > > If the whole DDT does make it into the cache, or onto an SSD > storing an extra copy of all pool metadata, then searching > for a particular entry in DDT would be faster. When you write > (or delete) and need to update the counters in DDT, or even > ultimately remove an unreferenced entry, then you benefit on > writes as well - you don't take as long to find DDT entries > (or determine lack thereof) for the blocks you add or remove. > > Or did I get your answer wrong? ;)
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. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
