the main line of how to calcule the probability here: YOU CAN´T HAVE A LOST OF INFORMATION! so you can´t allow the MIN(probability to fail) be the secured probability you MUST use the MAX(probability to fail) MAX(probability to fail) = 1 disk failed = 1 mirror failed got? 2011/1/31 Roberto Spadim <roberto@xxxxxxxxxxxxx>: > the question is: > how many mirrors you have? you don´t have a partial mirror (i didn´t > found it on raid documentation yet), or you have a working mirror or > you don´t have the mirror and must resync to have a running one > > raid10 = raid1 > but the raid1 devices are raid0 > if you put raid1 over raid0 or raid0 over raid1 is not a diference of > security. just a diference of how many time i will wait to resync the > raid1 mirror (a big raid0 you slower than smallers harddisks/ssd > devices) > > the question again: > how many mirrors you have? > > 2011/1/31 Keld Jørn Simonsen <keld@xxxxxxxxxx>: >> Top-posting... >> >> How is the raid0+1 problem of only 33 % survival for 2 disk with RAID10? >> >> I know for RAID10,F2 the implementation in Linux MD is bad. >> It is only 33 % survival, while it with a probably minor fix could be 66%. >> >> But how with RAID10,n2 and RAID10,o2? >> >> best regards >> keld >> >> >> On Mon, Jan 31, 2011 at 05:15:29PM -0200, Roberto Spadim wrote: >>> ok, but lost of a disk = problem with hardware = big problems = mirror failed >>> think about a 'disaster recover' system >>> you can?t lost the main data (you MUST have one 'primary' data source) >>> >>> raid1 don?t have ecc or anyother 'paged' data recover solution (it >>> have just all mirror resync) >>> >>> let?s get back a level... (inside hard disk) >>> if your hard disk have 2 heads, you have a raid0 inside you disk (got >>> the point?) >>> using your math, you should consider head problem (since it make the >>> real read of information) >>> >>> but at raid (1/0) software (firmware) level, you have devices (with >>> out without heads, can be memory or anyother type of adresseable >>> information souce, RAID0 = DEVICE for raid software/firmware, but you >>> have A DEVICE) >>> >>> for raid 1 you have mirrors(a copy of one primary device) >>> if software find 1bit of error inside this mirror(device), you lost >>> the full mirror, 1bit of fail = mirror fail!!!!! it?s not more sync >>> with the main(primary) data source!!!! >>> >>> got the problem? mirror will need a resync if any disk fail (check >>> what fail make you mirror to fail, but i think linux raid1 mirror fail >>> with any disk fail) >>> >>> if you have 4 mirrors you can loose 4 disks (1 disk fail = mirror >>> fail, 2 disk fail = mirror fail, 3 disk fail = mirror fail, any device >>> with fail inside a raid1 device will make the mirror to fail, got? you >>> can have good and bad disks on raid0, but you will have a mirror >>> failed if you have >=1 disk fail inside your raid0) >>> >>> got the point? >>> what?s the probability of your mirror fail? >>> if you use raid0 as mirror >>> any disk of raid0 failed = mirror failed got? >>> you can lose all raid0 but you have just 1 mirror failed! >>> >>> >>> could i be more explicit? you can?t make probability using bit, you >>> must make probability using mirror, since it?s you level of data >>> consistency >>> =] got? >>> >>> >>> 2011/1/31 Denis <denismpa@xxxxxxxxx>: >>> > 2011/1/31 Roberto Spadim <roberto@xxxxxxxxxxxxx>: >>> >> i think that partial failure (raid0 fail) of a mirror, is a fail >>> >> (since all mirror is repaired and resync) >>> >> the security is, if you lose all mirrors you have a device >>> >> so your 'secure' is the number of mirrors, not the number of disks ssd >>> >> or another type of device... >>> >> how many mirrors you have here: >>> >> raid0= 1,2(a) 3,4(b) >>> >> raid1=a,b >>> >> 1 mirror (a or b) >>> >> >>> >> and here: >>> >> raid1=1,2(a) 3,4(b) >>> >> raid0=ab >>> >> 1 mirror (a or b) >>> >> >>> >> let?s think about hard disk? >>> >> your hard disk have 2 disks? >>> >> why not make two partition? first partition is disk1, second partition is disk2 >>> >> mirror it >>> >> what?s your security? 1 mirror >>> >> is it security? normaly when a harddisk crash all disks inside it >>> >> crash but you is secury if only one internal disk fail... >>> >> >>> >> that?s the point, how many mirror? >>> >> the point is >>> >> with raid1+0 (raid10) we know that disks are fragments (raid1) >>> >> with raid0+1 we know that disks are a big disk (raid0) >>> >> the point is, we can?t allow that information stop, we need mirror to >>> >> be secured (1 is good, 2 better, 3 really better, 4 5 6 7...) >>> >> you can?t break mirror (not disk) to don?t break mirror have a second >>> >> mirror (raid0 don?t help here! just raid1) >>> >> >>> >> with raid10 you will repair smal size of information (raid1), here >>> >> sync will cost less time >>> >> with raid01 you will repair big size of information (raid0), here >>> >> sync will cost more time >>> > >>> > Roberto, to quite understend how better a raid 10 is over raid 01 you >>> > need to take down into a mathematical level: >>> > >>> > once I had the same doubt: >>> > >>> > "The difference is that the chance of system failure with two drive >>> > failures in a RAID 0+1 system with two sets of drives is (n/2)/(n - 1) >>> > where n is the total number of drives in the system. The chance of >>> > system failure in a RAID 1+0 system with two drives per mirror is 1/(n >>> > - 1). So, for example, using a 8 drive system, the chance that losing >>> > a second drive would bring down the RAID system is 4/7 with a RAID 0+1 >>> > system and 1/7 with a RAID 1+0 system." >>> > >>> > >>> > Another problem is that in the case of a failury of one disk ( in a >>> > two sets case), in a raid01 you will loose redundancy for ALL your >>> > data, while in a raid10 you will loose redundancy for 1/[(n/2 >>> > -1)/(n/2)], in the same case 1/4 of your data set. >>> > >>> > And also, in a raid 10 you will have o re-mirror just one disk in the >>> > case of a disk failure, in raid 01 you will have to re-mirror the >>> > whole failed set. >>> > >>> > -- >>> > Denis Anjos, >>> > www.versatushpc.com.br >>> > -- >>> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in >>> > the body of a message to majordomo@xxxxxxxxxxxxxxx >>> > More majordomo info at http://vger.kernel.org/majordomo-info.html >>> > >>> >>> >>> >>> -- >>> Roberto Spadim >>> Spadim Technology / SPAEmpresarial >>> -- >>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > > > -- > Roberto Spadim > Spadim Technology / SPAEmpresarial > -- Roberto Spadim Spadim Technology / SPAEmpresarial -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
