On Thu, Apr 05, 2012 at 09:54:45PM +1000, Russell Coker wrote:
On Thu, 5 Apr 2012, Craig Sanders <cas@taz.net.au> wrote:
ZFS can also do de-duping(**) but vast quantities of RAM & L2ARC required, on the order of 4-6GB RAM or more per TB of storage
http://configure.ap.dell.com/dellstore/config.aspx?oc=u421102au&c=au&l=en&s=...
If you have a need for 15k rpm SAS disks then you are limited to a maximum size of about 600G per disk. If you get a server like a Dell PowerEdge T110 then you have a maximum of 4 disks (1.8TB of RAID-5) and 32G of RAM. So using 7-11G of RAM to save some of that 1.8T of storage could be useful.
yeah, well, i'm totally unconvinced that 15k RPM SAS drives actually provide any noticable performance benefit that's even close to being worth the price/GB, even compared to the price of enterprise-quality 1,2, or 3TB 7200rpm SATA drives...and even more so when compared to consumer quality SATAs(*). especially when you can use SSDs for read and write caching of your drives (buffer your random writes through an SSD as with ZFS ZiL and they end up being mostly sequential writes). sure, you can use those SSDs to cache 15k drives too...but is the result significantly better than using the same SSDs to cache 7200rpm drives? sometimes price doesn't matter and the highest possible I/O performance at any price is required. those times are very rare. far more rare than the salesmen with the slick $150K+ SAN brochures would have you believe. and even then, IMO, you'd be better off using, say, 240GB SSDs rather than 300GB SAS drives - 20% less storage but many times the IOPS, for roughly the same price. even 500-ish GB SSDs aren't that much more than 600GB 15k SAS disks, about $860 for a 480GB Intel 520 vs about $575 for an IBM 15k 600GB SAS....80,000 IOPS vs what, maybe 1000? and an Intel 520 is far from the best performing SSD around. if you've got thousands to spend then a TB or two of SSD on a PCI-e card (no SATA or SAS bottleneck) beats any currently available SAS or SATA disk or SSD by a huge margin. you can also fit more 2.5" SSDs in a 1 or 2RU server than 3.5" SAS drives. if top performance is your requirement then, IMO, 15k drives are the wrong answer. (*) a large part of the point of RAID is that it is a Redundant Array of *Inexpensive* Disks. enterprise drives fail on that particular point. the disks are meant to be cheap and replacable commodity parts.
Of course if you want to use large SATA disks with SSD and other forms of cache then things are totally different.
true. and for de-duping, ZFS will use your L2ARC (e.g. SSD) as well as your ARC (RAM) for the dupe hash tables. I still don't think it's worthwhile in the general case. 8GB sticks are cheap enough now that I could upgrade my home server from 16GB to 32GB for not too much money...but even though one of my zpools has a LOT of duplicate data (rsync backups of linux systems) I still don't think it's worth the bother. i'd rather use that extra RAM for disk caching or for VMs. and upgrade the backup zpool from 4x1TB to 4x2TB. or just save the money and wait for the inevitable improvements :) craig -- craig sanders <cas@taz.net.au>