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
