> -----Original Message----- > From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid- > owner@xxxxxxxxxxxxxxx] On Behalf Of Keld Jørn Simonsen > Sent: Friday, May 06, 2011 8:28 AM > To: David Brown > Cc: linux-raid@xxxxxxxxxxxxxxx > Subject: Re: wish for Linux MD mirrored raid types > > On Fri, May 06, 2011 at 12:54:28PM +0200, David Brown wrote: > > On 06/05/2011 12:05, Jonathan Tripathy wrote: > > > > > >>>>RAID1 is traditionally a mirror only setup (ok, some RAID > > >>>>implementations may do some load-balancing of some sort). So a RAID1 > > >>>>with 4 disks is one data set copied onto 4 disks. Bandwidth is > roughly > > >>>>the same as a single disk (ignoring any load balancing). > > >>>>RAID10 is mirror and stripe. A RAID10 with 4 disks is similar to a 2 > > >>>>disk RAID0 (double bandwidth with data split in half across both > > >>>>disks), > > >>>>but with each disk having a mirror (which brings the total up to 4 > > >>>>drives). > > >>>> > > >>>>Additionally, a RAID1 disk (at least using MD) can be accessed just > > >>>>like > > >>>>a normal disk (good for recovery etc.) however a single disk out of > a > > >>>>RAID10 array is next to useless. > > >Just so I can sleep at night, is my understanding of RAID10 and RAID1 > > >above correct? > > > > > > > It's mostly right (assuming, of course, that /I/ am correct here...). > > > > RAID1 is traditionally a two-way mirror on two disks (or possibly on two > > other raid sets, as in raid0+1). So if you talk about RAID1 with 4 > > disks, you should probably qualify it more precisely - otherwise people > > will wonder what you mean, or think it is impossible (many other RAID1 > > solutions, hardware or software, don't support more than two-way > > mirrors). /I/ would certainly say that a 4-disk RAID1 is a four-way > > mirror as you described - but some people might think of a standard > > layout RAID1+0. > > > > As you say, RAID10,near on four disks is pretty much identical to > > RAID1+0 - i.e., a stripe of two normal RAID1 pairs. > > > > A single md RAID1 disk can be accessed like a normal disk, /if/ it uses > > metadata format 0.90 which is put at the end of the drive. If you have > > later metadata formats that are at the beginning, then that will cause > > trouble if you try to view the disk without using md. A single disk > > from a RAID10 is, as you say, useless without md. > > > > However, assuming your recovery PC supports md raid, then you can > > assembly your single RAID10 disk as a degraded RAID10 array. After all, > > it wouldn't be very redundant if you only had access to your data when > > all the disks were working! > > I agree with what David Brown says here. > > Just some further remarks: > > You can have Linux MD raid10 arrays with only 2 disks. > This is different from RAID1+0 that requires at least 4 disks. > > You can also with Linux MD have an odd number of disk, which is > impossible with RAID1+0. > > Linux MD raid10 is a kernel driver that handles everything in > one module, while RAID1+0 is handled in two drivers - raid0 over raid1. > The latter is called nested RAID, and the Linux MD raid10 is not a > nested RAID, while RAID1+0 is. > > With Linux MD raid10,near and superblock 0.90 you can also > boot it from old grub, lilo etc, as each partition can be seen as a normal > file system. > > As David pointed out, "a single disk out of a RAID10 array is next to > useless" is not true. Linux MD RAID10 is designed to always be able to > hold all data intact if one disk is out. You just need Linux MD raid10 > aware software. > > I apologise for being abrupt in my first answer. But the thing is that > I am getting tired of explaining and correcting all these misconceptions > again and again. And my proposal is an attempt to stop at least some > of the need for explaining, while also bringing the best practise forward > to > the users of our technology, for the benefit of the users. I think there are several more issues at hand, here, than your suggestion takes into consideration. Special Relativity, as the name suggests, is a special case of General Relativity, but no one, and I mean *NO ONE* ever approaches an SR problem by applying the mechanics of GR and letting the equations collapse along the way. Indeed, Einstein himself developed SR several years before tackling the much more complex problem of GR, the fact all the same physical processes are involved and the fact the postulates are identical notwithstanding. It doesn't really much matter whether the developers decide to implement RAID1 as a part of the same engine that delivers RAID10, or not. It also does not matter whether the admin can produce the exact same results at the code execution level by implementing a specific topology of RAID10 or not. OTOH, however, if there are any functional or operational differences between the implementation of RAID1 and an implementation of RAID10 that produces the same layout, then the two are simply not the same, at all. Period. The bottom line, however, is that even if RAID1 is at the code level just a specific implementation of RAID10, it still is a sufficiently distinct operational mode to warrant its own name, setting it aside from the more general RAID10 implementation. Aside from that, when I say, "My servers boot from 2 disk RAID1 arrays", everyone knows exactly what I mean without my having to go into more detail about the layout. For the most part, it doesn't really matter if I happened to create the array using a specific layout of RAID10. This has its greatest importance for someone inexperienced with RAID systems, who perhaps is setting up his very first mirrored array with a pair of disks. Having to try to learn all the ins and outs of RAID10 layouts just to be able to create a pair (or triplet) of mirrored disks is not appropriate. -- 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
