From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roger Heflin Subject: Re: new bottleneck section in wiki Date: Wed, 02 Jul 2008 16:55:37 -0500 Message-ID: <486BF959.9050202@gmail.com> References: <20080702155603.GA11156@rap.rap.dk> <20080702175117.GC12081@rap.rap.dk> <20080702182627.GA12614@rap.rap.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20080702182627.GA12614@rap.rap.dk> Sender: linux-raid-owner@vger.kernel.org To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= Cc: David Lethe , linux-raid@vger.kernel.org List-Id: linux-raid.ids Keld J=F8rn Simonsen wrote: > On Wed, Jul 02, 2008 at 01:08:04PM -0500, David Lethe wrote: >> >> And also the disk controllers, could these be bottlenecks? They typi= cally >> operate at 300 MB/s nominally, per disk channel, and presumably they >> then have a connection to the southbridge that is capable of handlin= g >> this speed. So for a 4-disk SATA-II controller this would be at leas= t >> 1200 MB/s or about 10 gigabit.=20 >> >> best regards >> keld >> ------------------- >> It is much more complicated than just saying what the transfer rates= are, especially in the world of blocking, arbitration, and unbalanced = I/O. =20 >=20 > Yes, that is understood, but I am only listing some potential > bottlenecs, of cause there may be more. >=20 >> Everything is a potential bottleneck. As I am under NDA with most o= f the controller vendors, then I can not provide specifics, but suffice= to say that certain cards with certain chipsets will max out at well u= nder published speeds. Heck, you could attach solid-state disks with r= andom I/O access time in the nanosecond range and still only get 150MB/= sec out of certain controllers, even on a PCIe X 16 bus.=20 >> >> BTW, there isn't a SATA-II controller in the planet that will delive= r 1200 MB/sec with 4 disk drives.=20 >=20 > Yes, but I think this is normally due to that the max transfer speed = per > disk is in the ballpark of 80-120 MB/s - which is less than half the > SATA-II max speed. And I think much of this slowdown comes from head = movement, > track-to-track, disk latency etc. I was of the impression, that when = the > transfer between the disk and the controller is going on, then the > transfer speed would be not far from the 300 MB/s max speed, eg for > 90 MB/s 1 TB disks that I bougth recently, or the faster 15000 RPM > disks, which give something like 120 MB/s. Actually enough sectors are only passing under the head at that 70-120M= B/second=20 rate, so when you add in the seeks and such things are a bit lower, but= you=20 aren't ever going to be able to exceed that given bit rate, and the bit= rate=20 changes from the inside of the disk to the outside of the disk (the out= side has=20 more sectors on it than the inside), in the disk data sheet's there is = usually a=20 range of bit rates listed showing this. Roger Roger -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html