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
