From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: misunderstanding of spare and raid devices? - and one question more Date: Fri, 01 Jul 2011 09:23:43 +0200 Message-ID: References: <4E0C5539.4030000@gmx.de> <4E0C5E47.5090604@anonymous.org.uk> <4E0C6CC4.3030506@turmel.org> <4E0C7196.1070307@gmx.de> <4E0C7B4B.7090404@turmel.org> <4E0C8685.3020806@gmx.de> <20110701072855.69ee763b@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110701072855.69ee763b@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 30/06/2011 23:28, NeilBrown wrote: > On Thu, 30 Jun 2011 16:21:57 +0200 Karsten R=F6mke = wrote: > >> Hi Phil >>> >>> If your CPU has free cycles, I suggest you run raid6 instead of rai= d5+spare. >>> >>> Phil >>> >> I started the raid 6 array and get: >> >> Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] >> md0 : active raid6 sde5[4] sdd5[3] sdc5[2] sdb2[1] sda3[0] >> 13759296 blocks level 6, 64k chunk, algorithm 2 [5/5] [UUUUU= ] >> [=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D>...] r= esync =3D 87.4% (4013184/4586432) finish=3D0.4min speed=3D20180K/sec > ^^^^^^ > Note: resync > >> >> when I started the raid 5 array I get >> >> md0 : active raid5 sdd5[4] sde5[5](S) sdc5[2] sdb2[1] sda3[0] >> 13759296 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_] >> [=3D>...................] recovery =3D 6.2% (286656/458643= 2) finish=3D0.9min speed=3D71664K/sec > ^^^^^^^^ > Note: recovery. > >> >> so I have to expect a three times less write speed - or is this calc= ulation >> to simple ? >> > > You are comparing two different things, neither of which is write spe= ed. > If you want to measure write speed, you should try writing and measur= e that. > > When you create a RAID5 mdadm deliberately triggers recovery rather t= han > resync as it is likely to be faster. This is why you see a missed de= vice and > an extra spare. I don't remember why it doesn't with RAID6. > What's the difference between a "resync" and a "recovery"? Is it that = a=20 "resync" will read the whole stripe, check if it is valid, and if it is= =20 not it then generates the parity, while a "recovery" will always=20 generate the parity? If that's the case, then one reason it might not do that with raid6 is=20 if the code is common with the raid5 to raid6 grow case. Then a=20 "resync" would leave the raid5 parity untouched, so that the set keeps=20 some redundancy, whereas a "recovery" would temporarily leave the strip= e=20 unprotected. -- 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