Date: Mon, 19 Sep 2011 12:00:11 -0700 (PDT) From: Jason Usher <jusher71@yahoo.com> To: freebsd-fs@freebsd.org Subject: Re: ZFS obn FreeBSD hardware model for 48 or 96 sata3 paths... Message-ID: <1316458811.88701.YahooMailClassic@web121208.mail.ne1.yahoo.com> In-Reply-To: <72A6ABD6-F6FD-4563-AB3F-6061E3DD9FBF@digsys.bg>
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--- On Sat, 9/17/11, Daniel Kalchev <daniel@digsys.bg> wrote: > There is not single magnetic drive on the market that can > saturate SATA2 (300 Mbps), yet. Most can't match even SATA1 > (150 MBps). You don't need that much dedicated bandwidth for > drives. > If you intend to have 48/96 SSDs, then that is another > story, but then I am doubtful a "PC" architecture can handle > that much data either. Hmmm... I understand this, but is there not any data that might transfer from multiple magnetic disks, simultaneously, at 6GB, that could periodically max out the card bandwidth ? As in, all drives in a 12 drive array perform an operation on their built-in cache simultaneously ? I know the spinning disks themselves can't do it, but there is 64 MB of cache on each drive, and that can run at 6G ... this doesn't ever happen ? Further, the cards I use will be the same regardless - the number of PCIe lanes is just a different motherboard choice at the front end, and only adds a marginal extra cost (assuming there _IS_ a 112+ lane mobo around) ... so why not ? > Memory is much more expensive than SSDs for L2ARC and if > your workload permits it (lots of repeated small reads), > larger L2ARC will help a lot. It will also help if you have > huge spool or if you enable dedup etc. Just populate as much > RAM as the server can handle and then add L2ARC > (read-optimized). That's interesting (the part about dedup being assisted by L2ARC) ... what about snapshots ? If we run 14 or 21 snapshots, what component is that stressing, and what structures would speed that up ? Thanks a lot.
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