On Wed, Nov 23 2016, Diego Guella wrote: > (2nd attempt: the previous one didn't make it) > Hi, > > I am using linux raid1 for a double-purpose: redundancy and backup. > > I have a raid1 array of 5 disks, 3 of which are kept for backup purposes. > Let's call disks A, B, C, D, E. > Disks A and B are _always_ connected to the system. > Disks C, D, E are backup disks. > Here follows a description of how I use the backup disks. > This morning I connect disk C, and let it resync. > Tomorrow morning, I shut down the system, remove disk C and keep it away > as a daily backup. > I connect the next disk (D), then start up the system. > Linux raid1 recognizes the "old" disk and does not allow it to enter the > array (this is evidenced by system logs). > I then add disk D to the array, and let it resync. So this would be a full resync - right? > The next day, I connect the next disk (E), and so on, rotating them. > The "connect and disconnect" is always performed when the system is > powered off, although sometimes I hot-connect the disk with the system > already powered up. > The purpose of this is to have an emergency backup: I can disconnect ALL > disks from the system and connect only one of the daily backups, going > "back to the past"(TM). > > This array has a write-intent bitmap, in order to speed up the resync > (it is a 4TB array, and sometimes it needs nearly 20 hours to resync > without bitmaps due to system load). > > This worked flawlessly (for some years) until some days ago, when the > array suffered a strange inconsistency, and the filesystem nearly gone > nuts in about 20 minutes of uptime. I will elaborate more on this > later. Did you ever test your backups? > > Since that problem happened, some questions come to my mind: > What raid1 bitmaps allow me to do? - accelerate resync after a crash. - accelerate recovery when you remove a drive and re-add it. > Can they record _correctly_ the state of multiple removed disks, in > order to overwrite only out-of-sync chunks of multiple removed disks? All that is recorded is the set of regions which have been written to since the array was last in a non-degraded state. > In other words, am I allowed to do what I described above? If the recovery that happened when you swapped drives was not a full recovery, then probably not. > If not, can I change something in my actions in order to have a daily > backup using raid1? I wrote something about this a few years ago... http://permalink.gmane.org/gmane.linux.raid/35074 or this thread http://www.spinics.net/lists/raid/msg35532.html NeilBrown > > > System details: > # cat /etc/debian_version > 6.0.10 > # mdadm --version > mdadm - v3.1.4 - 31st August 2010
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