> After modifications that I hope are corrections, I think the post > should look like this: > > The rule-of-thumb is 270 bytes/DDT entry, and 200 bytes of ARC for > every L2ARC entry. > > DDT doesn't count for this ARC space usage > > E.g.: I have 1TB of 4k blocks that are to be deduped, and it turns out > that I have about a 5:1 dedup ratio. I'd also like to see how much ARC > usage I eat up with a 160GB L2ARC. > > (1) How many entries are there in the DDT: > > 1TB of 4k blocks means there are 268million blocks. However, at a 5:1 > dedup ratio, I'm only actually storing 20% of that, so I have about 54 > million blocks. Thus, I need a DDT of about 270bytes * 54 million =~ > 14GB in size > > (2) My L2ARC is 160GB in size, but I'm using 14GB for the DDT. Thus, I > have 146GB free for use as a data cache. 146GB / 4k =~ 38 million > blocks can be stored in the > remaining L2ARC space. However, 38 million files takes up: 200bytes * > 38 million =~ 7GB of space in ARC. > > Thus, I better spec my system with (whatever base RAM for basic OS and > cache and application requirements) + 14G because of dedup + 7G > because of L2ARC.
Thanks, but one more ting: Add some tuning parameters, such as "set zfs:zfs_arc_meta_limit = somevalue in /etc/system" to help zfs use more memory for its metadata (like the DDT), as it won't use more than (RAM-1GB)/4 by default Vennlige hilsener / Best regards roy -- Roy Sigurd Karlsbakk (+47) 97542685 r...@karlsbakk.net http://blogg.karlsbakk.net/ -- I all pedagogikk er det essensielt at pensum presenteres intelligibelt. Det er et elementært imperativ for alle pedagoger å unngå eksessiv anvendelse av idiomer med fremmed opprinnelse. I de fleste tilfeller eksisterer adekvate og relevante synonymer på norsk. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
