Date: Tue, 20 Sep 2011 11:45:11 -0700 From: Freddie Cash <fjwcash@gmail.com> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS obn FreeBSD hardware model for 48 or 96 sata3 paths... Message-ID: <CAOjFWZ5mZc1%2BOcWao989JQgUCnABKLu=ijbwJw%2BzFXvzmi3c1A@mail.gmail.com> In-Reply-To: <alpine.GSO.2.01.1109201055160.26410@freddy.simplesystems.org> References: <alpine.GSO.2.01.1109171451500.1973@freddy.simplesystems.org> <1316459220.35419.YahooMailClassic@web121209.mail.ne1.yahoo.com> <CAOjFWZ6Y4PbAaZzMxyzZHmtrNBJzOK8GMVJL0GP0PMdwj9SeXg@mail.gmail.com> <alpine.GSO.2.01.1109191613520.7097@freddy.simplesystems.org> <CAOjFWZ5YG5raCwvE-kZQ9wjzYvxqCRBPFy47FWOsN1Yjoga1Bw@mail.gmail.com> <alpine.GSO.2.01.1109201055160.26410@freddy.simplesystems.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Tue, Sep 20, 2011 at 9:01 AM, Bob Friesenhahn < bfriesen@simple.dallas.tx.us> wrote: > On Mon, 19 Sep 2011, Freddie Cash wrote: > >> >> L2ARC has extreme bandwidth limitations as compared with RAM. Be careful >> what you wish for. >> >> For writes (7 MBps, I believe); there shouldn't be any limits on the >> reads. >> > > If (for example) an SSD is used with a 200MB/s read rate for the L2ARC, > then the L2ARC is limited to 200MB/s (as compared with perhaps 10GB/s or > 20GB/s for RAM). > Ah, yes, obviously it's limited by the hardware, but so is the pool. :) I meant there's no artificial limits on reads from an L2ARC device, or writes to a ZIL device. In contrast to the write throttling for the L2ARC device. > The L2ARC is really all about eliminating the access latency of > rotating-rust but any device will provide far less bandwidth than system > RAM. > Yes, L2ARC is definitely slower than RAM. But properly selected/configured L2ARC will be a heck of a lot faster/lower latency than the pool. Hence the ordering I gave originally: ARC -> L2ARC -> pool -- Freddie Cash fjwcash@gmail.com
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CAOjFWZ5mZc1%2BOcWao989JQgUCnABKLu=ijbwJw%2BzFXvzmi3c1A>