@Phillip Susi / comment #23: Did you actually read what I wrote? :) I was *NOT* advocating "backup" by having multiple RAID disks constantly connected to the array and in sync. It is completely obvious to me that a hot running copy of data is NOT a backup. I was advocating the following procedure: 1. Connect disk 2. Wait until it is synced into the array. 3. Shutdown the machine 4. *DISCONNECT* the disc from the machine and consider the completely offline disk as a backup.
This is a backup because the disk is physically disconnected from the machine. It is much better than a rsync/cp, because it provides a *coherent* copy since all modifications to the data which happen during the copying process are also applied on the backup. With rsync/cp, files which are modified *after* they have already been copied are not up to date in the backup, and for applications which store data in multiple files (which *many* programs do), their data would be corrupt in such a case. It is relevant to this bugtracker entry because it shows that using multiple kinds of disks in a RAID1, such as SATA+USB, is a common desire and not some exotic border case. Or can you name any other kind of non-exotic, non-beta (such as btrfs) backup mechanism which can copy data while the system is in use without breaking its coherency? :) -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/320638 Title: hot-add/remove in mixed (IDE/SATA/USB/SD-card/...) RAIDs with device mapper on top => data corruption (bio too big device md0 (248 > 240)) Status in The Linux Kernel: Confirmed Status in mdadm - Tool for managing linux software RAID arrays.: Confirmed Status in debian-installer package in Ubuntu: Invalid Status in linux package in Ubuntu: Won't Fix Status in mdadm package in Ubuntu: Confirmed Status in ubiquity package in Ubuntu: Invalid Bug description: Problem: md changes max_sector setting of an already running and busy md device, when a (hotplugable) device is added or removed. However, the device mapper and filesystem layer on top of the raid can not (always?) cope with that. Observations: * "bio too big device mdX (248 > 240)" messages in the syslog * read/write errors (some dropped silently, no noticable errors reported during operation, until things like dhcpclient looses its IP etc.) Expected: Adding and removing members to running raids (hotplugging) should not change the raid device characteristics. If the new member supports only smaller max_sector values, buffer and split the data steam, until the raid device can be set up from a clean state with a more appropriate max_sector value. To avoid buffering and splitting in the future, md could save the smallest max_sector value of the known members in the superblock, and use that when setting up the raid even if that member is not present. Note: This is reproducible in much more common scenarios as the original reporter had (e.g. --add a USB (3.0 these days) drive to an already running SATA raid1 and grow the number of devices). Fix: Upsteam has no formal bug tracking, but a mailing list. The response was that finally this needs to be "fixed [outside of mdadm] by cleaning up the bio path so that big bios are split by the device that needs the split, not be the fs sending the bio." However, in the meantime mdadm needs to saveguard against the date corruption: > > [The mdadm] fix is to reject the added device [if] its limits are > > too low. > > Good Idea to avoid the data corruption. MD could save the > max_sectors default limit for arrays. If the array is modified and the new > limit gets smaller, postpone the sync until the next assembe/restart. > > And of course print a message if postponing, that explains when --force would be save. > What ever that would be: no block device abstraction layer (device mapper, lvm, luks,...) > between an unmounted? ext, fat?, ...? filesystem and md? As upsteam does not do public bug tracking, the status and rememberence of this need remains unsure though. --- This is on a MSI Wind U100 and I've got the following stack running: HDD & SD card (USB card reader) -> RAID1 -> LUKS -> LVM -> Reiser Whenever I remove the HDD from the Raid1 > mdadm /dev/md0 --fail /dev/sda2 > mdadm /dev/md0 --remove /dev/sda2) for powersaving reasons, I cannot run any apt related tools. > sudo apt-get update [...] Hit http://de.archive.ubuntu.com intrepid-updates/multiverse Sources Reading package lists... Error! E: Read error - read (5 Input/output error) E: The package lists or status file could not be parsed or opened. Taking a look at the kernel log shows (and many more above): > dmesg|tail [ 9479.330550] bio too big device md0 (248 > 240) [ 9479.331375] bio too big device md0 (248 > 240) [ 9479.332182] bio too big device md0 (248 > 240) [ 9611.980294] bio too big device md0 (248 > 240) [ 9742.929761] bio too big device md0 (248 > 240) [ 9852.932001] bio too big device md0 (248 > 240) [ 9852.935395] bio too big device md0 (248 > 240) [ 9852.938064] bio too big device md0 (248 > 240) [ 9853.081046] bio too big device md0 (248 > 240) [ 9853.081688] bio too big device md0 (248 > 240) $ sudo mdadm --detail /dev/md0 /dev/md0: Version : 00.90 Creation Time : Tue Jan 13 11:25:57 2009 Raid Level : raid1 Array Size : 3871552 (3.69 GiB 3.96 GB) Used Dev Size : 3871552 (3.69 GiB 3.96 GB) Raid Devices : 2 Total Devices : 1 Preferred Minor : 0 Persistence : Superblock is persistent Intent Bitmap : Internal Update Time : Fri Jan 23 21:47:35 2009 State : active, degraded Active Devices : 1 Working Devices : 1 Failed Devices : 0 Spare Devices : 0 UUID : 89863068:bc52a0c0:44a5346e:9d69deca (local to host m-twain) Events : 0.8767 Number Major Minor RaidDevice State 0 0 0 0 removed 1 8 17 1 active sync writemostly /dev/sdb1 $ sudo ubuntu-bug -p linux-meta dpkg-query: failed in buffer_read(fd): copy info file `/var/lib/dpkg/status': Input/output error dpkg-query: failed in buffer_read(fd): copy info file `/var/lib/dpkg/status': Input/output error [...] Will provide separate attachements. 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