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
