Re: Two-disk RAID5?
Sorry, I couldn't find a diplomatic way to say you're completely wrong. We don't necessarily expect a diplomatic way, but a clear and intelligent one would be helpful. In two-disk RAID5 which is it? 1) The 'parity bit' is the same as the datum. 2) The parity bit is the complement of the datum. 3) It doesn't work at a bit-wise level. Many of us feel that RAID5 looks like: parity = data[0]; for (i=1; i ndisks; ++i) parity ^= data[i]; which implies (1). It could easily be (2) but merely saying it's not data, it's parity doesn't clarify matters a great deal. But I'm pleased my question has stirred up such controversy! John - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Two-disk RAID5?
On Friday May 5, [EMAIL PROTECTED] wrote: Sorry, I couldn't find a diplomatic way to say you're completely wrong. We don't necessarily expect a diplomatic way, but a clear and intelligent one would be helpful. In two-disk RAID5 which is it? 1) The 'parity bit' is the same as the datum. Yes. 2) The parity bit is the complement of the datum. No. 3) It doesn't work at a bit-wise level. No. Many of us feel that RAID5 looks like: parity = data[0]; for (i=1; i ndisks; ++i) parity ^= data[i]; Actually in linux/md/raid5 it is more like parity = 0 for (i=0; i ndisks; ++i) parity ^= data[i]; which has exactly the same result. (well, it should really be ndatadisks, but I think we both knew that was what you meant). which implies (1). It could easily be (2) but merely saying it's not data, it's parity doesn't clarify matters a great deal. But I'm pleased my question has stirred up such controversy! A bit of controversy is always a nice way to pass those long winter nights only it isn't winter anywhere at the moment :-) NeilBrown - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Two-disk RAID5?
On Thu, May 04, 2006 at 09:18:57PM -0400, Bill Davidsen wrote: Erik Mouw wrote: No, the other way around: RAID1 is a special case of RAID5. No it isn't. If you have N drives in RAID1 you have N independent copies of the data and no parity, there's just no corresponding thing in RAID5, which has one copy of the data, plus parity. There is no special case, it just doesn't work that way. Set N2 and report back. Just write out the formulas and it becomes obvious. Sorry, I couldn't find a diplomatic way to say you're completely wrong. I guess we have to agree to disagree. Erik -- +-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 -- | Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Two-disk RAID5?
Erik Mouw wrote: On Wed, Apr 26, 2006 at 03:22:38PM -0400, Jon Lewis wrote: On Wed, 26 Apr 2006, Jansen, Frank wrote: It is not possible to flip a bit to change a set of disks from RAID 1 to RAID 5, as the physical layout is different. As Tuomas pointed out though, a 2 disk RAID5 is kind of a special case where all you have is data and parity which is actually also just data. No, the other way around: RAID1 is a special case of RAID5. No it isn't. If you have N drives in RAID1 you have N independent copies of the data and no parity, there's just no corresponding thing in RAID5, which has one copy of the data, plus parity. There is no special case, it just doesn't work that way. Set N2 and report back. Sorry, I couldn't find a diplomatic way to say you're completely wrong. -- bill davidsen [EMAIL PROTECTED] CTO TMR Associates, Inc Doing interesting things with small computers since 1979 - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Two-disk RAID5?
John Rowe wrote: I'm about to create a RAID1 file system and a strange thought occurs to me: if I create a two-disk RAID5 array then I can grow it later by the simple expedient of adding a third disk and hence doubling its size. Is there any real down-side to this, such as performance? Alternatively is it likely that mdadm will soon be able to convert a RAID1 pair to RAID5 any time soon? (Just how different are they anyway? Isn't the RAID4/5 checksum just an OR?) I think it works, I just set up a little test case with two 20MB files and loopback mount. The mdadm seems to work, the mke2fs seems to work, the f/s is there. Please verify, this system is a bit (okay a bunch) hacked. -- bill davidsen [EMAIL PROTECTED] CTO TMR Associates, Inc Doing interesting things with small computers since 1979 - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Two-disk RAID5?
On Wed, Apr 26, 2006 at 03:22:38PM -0400, Jon Lewis wrote: On Wed, 26 Apr 2006, Jansen, Frank wrote: It is not possible to flip a bit to change a set of disks from RAID 1 to RAID 5, as the physical layout is different. As Tuomas pointed out though, a 2 disk RAID5 is kind of a special case where all you have is data and parity which is actually also just data. No, the other way around: RAID1 is a special case of RAID5. The parity of RAID5 with n disks is contructed like[1]: parity = disk1 XOR disk2 XOR ... XOR disk n-1 With n = 2, this reduces to: parity = disk1 XOR nothing = disk1 Which is just mirroring, which we usually call RAID1. Seems kind of like a RAID1 with extra overhead. I don't think I've ever heard of a RAID5 implementation willing to handle 3 drives though. Our own RAID recovery tools can handle that just fine. Erik [1] Yes, there's also an algorithm to select which disk is used for parity for what block, but that doesn't change the way *how* parity is calculated. -- +-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 -- | Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Two-disk RAID5?
On Wed, 26 Apr 2006, John Rowe wrote: I'm about to create a RAID1 file system and a strange thought occurs to me: if I create a two-disk RAID5 array then I can grow it later by the simple expedient of adding a third disk and hence doubling its size. No. When one of the 2 drives in your RAID5 dies, and all you have for some blocks is parity info, how will the missing data be reconstructed? You could [I suspect] create a 2 disk RAID5 in degraded mode (3rd member missing), but it'll obviously lack redundancy until you add a 3rd disk, which won't add anything to your RAID5 storage capacity. -- Jon Lewis | I route Senior Network Engineer | therefore you are Atlantic Net| _ http://www.lewis.org/~jlewis/pgp for PGP public key_ - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Two-disk RAID5?
No. When one of the 2 drives in your RAID5 dies, and all you have for some blocks is parity info, how will the missing data be reconstructed? You could [I suspect] create a 2 disk RAID5 in degraded mode (3rd member missing), but it'll obviously lack redundancy until you add a 3rd disk, which won't add anything to your RAID5 storage capacity. IMO if you have a 2-disk raid5, the parity for each block is the same as the data. There is performance drop as I suspect md isn't smart enough to read data from both disks, but that's all. When one disk fails, the (lone) parity block is quite enough to reconstruct. With XOR parity, you can always assume any amount of additional disks full of zero, it doesn't really change the algorithm. (maybe mdadm could/can change a raid-1 into raid5 by just changing the superblocks, for the purpose of expanding into more disks..) - tuomas - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
RE: Two-disk RAID5?
On Wed, 26 Apr 2006, Jansen, Frank wrote: It is not possible to flip a bit to change a set of disks from RAID 1 to RAID 5, as the physical layout is different. As Tuomas pointed out though, a 2 disk RAID5 is kind of a special case where all you have is data and parity which is actually also just data. Seems kind of like a RAID1 with extra overhead. I don't think I've ever heard of a RAID5 implementation willing to handle 3 drives though. I suspect I should have just kept out of this, and waited for someone like Neil to answer authoratatively. So...Neil, what's the right answer to Tuomas's 2 disk RAID5 question? :) -- Jon Lewis | I route Senior Network Engineer | therefore you are Atlantic Net| _ http://www.lewis.org/~jlewis/pgp for PGP public key_ - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
RE: Two-disk RAID5?
On Wednesday April 26, [EMAIL PROTECTED] wrote: I suspect I should have just kept out of this, and waited for someone like Neil to answer authoratatively. So...Neil, what's the right answer to Tuomas's 2 disk RAID5 question? :) .. and a deep resounding voice from on-high spoke and in it's infinite wisdom it said yeh, whatever The data layout on a 2disk raid5 and a 2 disk raid1 is identical (if you ignore chunksize issues (raid1 doesn't need one) and the superblock (which isn't part of the data)). Each drive contains identical data(*). Write throughput to a the r5 would be a bit slower because data is always copied in memory first, then written. Read through put would be largely the same if the r5 chunk size was fairly large, but much poorer for r5 if the chunksize was small. Converting a raid1 to a raid5 while offline would be quite straight forward except for the chunksize issue. If the r1 wasn't a multiple of the chunksize you chose for r5, then you would lose the last fraction of a chunk. So if you are planning to do this, set the size of your r1 to something that is nice and round (e.g. a multiple of 128k). Converting a raid1 to a raid5 while online is something I have been thinking about, but it is not likely to happen any time soon. I think that answers all the issues. NeilBrown (*) The term 'mirror' for raid1 has always bothered me because a mirror presents a reflected image, while raid1 copies the data without any transformation. With a 2drive raid5, one drive gets the original data, and the other drive gets the data after it has been 'reflected' through an XOR operation, so maybe a 2drive raid5 is really a 'mirrored' pair Except that the data is still the same as XOR with 0 produces no change. So, if we made a tiny change to raid5 and got the xor operation to start with 0xff in every byte, then the XOR would reflect each byte in a reasonable meaningful way, and we might actually get a mirrored pair!!! But I don't think that would provide any real value :-) - To unsubscribe from this list: send the line unsubscribe linux-raid in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
