Date: Sat, 16 Feb 2013 20:05:53 +0100 From: "C. P. Ghost" <cpghost@cordula.ws> To: Jens Schweikhardt <schweikh@schweikhardt.net> Cc: questions@freebsd.org Subject: Re: Should I bother with a gvinum stripe when using a pair of SSDs? Message-ID: <CADGWnjXErEgEwYxX1BircMHkryirnD%2Bkox2=WrGMRm9cJpVvCA@mail.gmail.com> In-Reply-To: <20130216183449.GD3070@schweikhardt.net> References: <20130216144751.GC3070@schweikhardt.net> <CADGWnjVJqm_s%2BWzF9_=Wt6tCg8kODbZor5-JXe-wdbiTBaZf=Q@mail.gmail.com> <20130216183449.GD3070@schweikhardt.net>
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On Sat, Feb 16, 2013 at 7:34 PM, Jens Schweikhardt <schweikh@schweikhardt.net> wrote: > On Sat, Feb 16, 2013 at 04:44:06PM +0100, C. P. Ghost wrote: > # On Sat, Feb 16, 2013 at 3:47 PM, Jens Schweikhardt > # <schweikh@schweikhardt.net> wrote: > # > hello, world\n > # > > # > currently the only gvinum partition on my home system is a stripe for /home > # > across two Velociraptor HDDs. I'm thinking of replacing the HDDs with a > # > pair of SSDs. I was thinking of reducing complexity and in the migration > # > possibly no longer use gvinum at all--one less thing to configure and worry > # > about. > # > > # > * Would gvinum striping bring any speed advantage with a pair of SSDs? > # > * Or am I hitting other limits so that striping SSDs is a waste anyway? > # > * Should I finally take the plunge and acquaint myself with ZFS? > # > > # > System has 4GB RAM in an ASUS P5Q3 Deluxe with SATA II. It appears to me > # > that SATA II with 300MB/s is maxed out by a single SSD and striping it > # > will not improve r/w throughput. Is my simplistic reasoning correct? > # > # Jens, > # > # as always it depends on what you're trying to achieve: > # - max speed / lower latency? > # - max storage? > # - max redundancy? > # - max run-time-to-data-loss? > # > # Your choice of SSD probably means you'd like to reduce latency > # and maximize data throughput. > > Exactly, when I started vith vinum many years ago in the magnetic HD > age, striping with vinum gave me almost factor 2 in r/w speed as > measured with dd. (I do backups regularly to other media, so data > loss protection is not my primary concern). > > I realize that maximum SSD speeds as advertised by vendors and tests > (e.g. 520MB/s for contemporary top notch SSDs) may only be reached > under certain conditions far away from my normal usage, which is > re-building worlds and kernels and ports on a daily basis. So if for > my realworld working set a single SSD can deliver 300MB/s, striping > with vinum just might get me a factor 2 again to 600MB/s across > two SSDs. Then it would be worthwile to keep gvinum. > > Does that make sense? My understanding of SSD and SATA capabilities > may however be completely dreamed up... Assuming the right cables etc.., I *guess* the limiting factor would ultimately be the (AHCI-)SATA controller itself, or even the bus to which it is attached. Add to this that embedded DMA controllers may compete for the bus, limiting transfer rates even more. I don't know how such a setup scales; that's way too system dependent. If you have hard requirements w.r.t. latency and throughput, you'll ultimately have to run some real world tests on the target system. > Regards, > > Jens > -- > Jens Schweikhardt http://www.schweikhardt.net/ > SIGSIG -- signature too long (core dumped) -cpghost. -- Cordula's Web. http://www.cordula.ws/
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