From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: MD performance options: More =?windows-1252?Q?CPU=92s_or_?= =?windows-1252?Q?more_Hz=92s=3F?= Date: Wed, 04 Nov 2009 09:05:45 -0500 Message-ID: <4AF18A39.6010404@tmr.com> References: <66781b10911040149q165edf1s94a86f179f9af9fc@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <66781b10911040149q165edf1s94a86f179f9af9fc@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: mark delfman Cc: Linux RAID Mailing List List-Id: linux-raid.ids mark delfman wrote: > Hi... I am wondering if anyone can offer some advice on MD performanc= e > related to CPU (speed and or cores). The basic question (probably to= o > basic) is =93for more MD performance are you better with more cpu=92s= or a > faster single cpu=94. (in an ideal world we would have lots of very > fast CPUs, but given we never have enough money....). > > Is there any grounding to the following logic: > > Presuming that a RAID0 will deliver 1.5GBsec and a RAID6 circa > 700MBsec, I am guessing there are many complex reasons for the > difference, but one of the more obvious being the need for the CPU to > perform all the necessary R6 overheads. > > If we look at a single RAID 6 configuration, then I am guessing if we > increase the speed of the CPU from eg 2.0GHz to 2.8GHz (quad core > xeon) then the RAID6 calculations would be faster? Would other > overheads also be faster and if so is there any know relationship > between CPU Hz and MD performance (maybe even rough rule of thumb eg > double cpu Hz and increase R6 performance by 20% etc) > > If however I start to think of multiple RAID6 configurations maybe vi= a > iSCSI etc... then I wonder if MD would be better served with more CPU= s > instead... for example 2 x Quad core 2.0GHz xeons instead of 1 x 2.8. > This theory is dependent on linux / md effectively parallel > processing the overheads and I have no knowledge in this area... henc= e > the question. > > Any thoughts anyone? > =20 Your logic is correct, but it implies that you expect "faster=20 calculation" to mean "faster write performance," and that is usually=20 true only at very low or very high write loads. Very low, because you get the io queued a few ns faster. Since the disk= =20 still has to do the write, this is essentially meaningless. Very high, because with many drives and a huge write volume you could,=20 in theory, start having CPU issues. I suggest that before you worry over much on that, you look at CPU usag= e=20 at idle and then at gradually increasing write load, and look at system= =20 time vs. GB/sec to see if you are actually getting anywhere near the=20 limit, or even up enough to notice. In my look at this a few years ago = I=20 didn't see any issues, but that was with only eight drives in the array= =2E=20 Measurement is always good, but in general drive performance is the=20 limiting factor rather than CPU. You didn't ask: if you use ext[34] filesystems, there is a gain to be=20 had from tuning the stripe and stride parameters, at least for large=20 sequential io. My measurements were on 2.6.26, so are out of date, but=20 less head motion is always better. Others may have more experience, other than load testing the array has=20 never been stressed, performance of backup servers is less important=20 than reliability. --=20 Bill Davidsen Unintended results are the well-earned reward for incompetence. -- 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