On Wed, Jul 18, 2012 at 2:17 AM, Bob Proulx <b...@proulx.com> wrote: > Muhammad Yousuf Khan wrote: >> Gary Dale wrote: >> > Muhammad Yousuf Khan wrote: >> >> or just one thing if you please explain these commands. i think my >> >> confusion will be cleared >> >> mdadm --assemble /dev/md1 /dev/hda1 >> >> mdadm --assemble /dev/md3 /dev/hda3 >> > >> > The first set of commands should give you error messages unless >> > the disk devices are actually part of the array. Assemble normally >> > doesn't need to know the disk partitions that form the array, but >> > you can list them. This could be done if the superblocks were >> > damaged, for example. You need to get the order right in that case >> > or the device may not be readable. > > The first command assembles an array using only one of the devices. > This emits a "degraded" array of only one of the two mirrored devices. > This is useful in specialized instances. It isn't the typical thing > to do. It is sometimes useful to create a degraded array with one > disk now and then later add a second disk. This appears to be what > you are doing across the two sets of commands. > > If you assemble an array with only one disk then there is no > redundancy. If you assemble an array with only one disk then the > array will be marked degraded and later when a second disk is added > then the array must be sync'd in order to bring it into normal > operation. That sync'ing process may take a long time. > >> > The normal command would be just mdadm --assemble /dev/md1. The >> > alternative would be to relist all the constituent devices. > > Agreed. Assembling the array with a single disk into a degraded mode > isn't typical. But it is useful for recovery data from a failed disk > on a second system used for data recovery. > >> >> mdadm --add /dev/md1 /dev/hdb1 >> >> mdadm --add /dev/md3 /dev/hdb3 >> >> >> >> what is the difference in both command thanks. any help will be highly >> >> appreciated. >> > >> > The second set of commands should give you error messages if the disk >> > devices are already part of the array. Add puts a new drive into the array. >> > The drive will be assumed to be dirty and a re-syncing operation will >> > commence. > > Yes. But by the combination of the two sets of commands: > >> >> mdadm --assemble /dev/md1 /dev/hda1 >> >> mdadm --assemble /dev/md3 /dev/hda3 >> >> mdadm --add /dev/md1 /dev/hdb1 >> >> mdadm --add /dev/md3 /dev/hdb3 > > That appears to be a less efficient way of doing this: > > mdadm --assemble /dev/md1 /dev/hda1 /dev/hdb1 > mdadm --assemble /dev/md3 /dev/hda3 /dev/hdb3 > > If you have two disks that you are trying to assemble into a RAID1 > mirror then you should assemble both devices together at once so that > the raid array may remain in sync. > > Let me guess... If you are recovering a single disk from a two disk > mirror but with one disk failed onto a second machine then (and pretty > much only then, no other time) would the combination of commands make > sense to me. > >> just one last question what do you mean by build. do you mean the sync >> which shows in "cat /proc/mdstat" and show the sync status b/w the two >> partition. does this called build? > > I went looking for that "build" you referenced: > > Gary Dale wrote in a previous message: >> Assemble builds the arrays based on the information contained in the >> superblock. > > "Build" and "assemble" are very similar in meaning in that context. > An array must first be "created". After an array has been created > then at future system boot times it will only be assembled (or built) > and not created again. "Create" zeros data and starts a clean empty > block of data with any previous data destroyed. "Assemble" or "build" > does not destroy data but activates an existing array with existing > data and makes the existing data available. > > Bob
Thanks you :) Bob and Gary -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: http://lists.debian.org/CAGWVfM=wwh2_uojxte2smra8ulfqtfmuhbfbptmdmu_omkk...@mail.gmail.com
