Hi Neil, I should probably elaborate a little more on my envisioned setup. I'd like to setup a RAID array using shared disks, then run LVM on top of the RAID array to carve out logical volumes. The logical volumes would be used by one node at a time (no data sharing). For both nodes to access the same LVM metadata, they must both activate the RAID array. It seemed like someone had done something similar here: http://marc.theaimsgroup.com/?l=linux-raid&m=98834623209225&w=2 Concerning the checksums, I was referring to the checksum output when I run mdadm --examine on a RAID disk. I thought that was a checksum of all the data blocks of the disk, but I guess not? So, basically the only time you can guarantee a RAID array is consistent is once it's been deactivated and the dirty flag cleared in the superblock? Thanks, -Kai -----Original Message----- From: Neil Brown [mailto:neilb@cse.unsw.edu.au] Sent: Sunday, May 25, 2003 3:57 AM To: Kai-Min Sung Cc: linux-raid@vger.kernel.org Subject: Re: cluster RAID On Sunday May 25, k@kaisung.com wrote: > Hi, > I have a shared storage environment (2 disks accessible by 2 > nodes through iSCSI) and am trying to assemble the same RAID-1 array on > both nodes. Whenever I try to assemble the RAID-1 array on the second > node, it always begins reconstructing the mirror. My guess for why it's > doing this is that after the first node assembles the array, it marks a > dirty flag in the RAID metadata blocks on disk. (It only resets the > dirty flag when it deactivates the array). When the second node tries to > assemble the same array, it reads the metadata blocks and sees that it > is dirty. Then it proceeds with reconstruction. My question is does > reconstruction happen, simply because the dirty flag is set? Why doesn't > it first check if the checksums on all the mirror disks match (i.e. the > array is consistent) and bypass reconstruction? Btw, I plan for both > nodes to be accessing different partitions in the array, so there > shouldn't be any synchronization problems. Also, I'm using mdadm-1.2.0 > for my testing. What you are doing doesn't really make sense (at least not to me). Having two hosts both trying to control a raid1 array cannot work as neither host can make an guarantees about consistancy. If you plan for both nodes to be accessing different partitions on the array, why not be up-front about that and have two different raid1 arrays. e.g. If your two drives are sda and sdb, then partition them into sda1, sda2, sdb1, sdb2 and then make md0 on node X from sda1 and sdb1, and md3 (or whatever) on node Y from sda2 and sdb2. To answer your question - yes, the second node reconstucts because the super block is marked dirty. I'm not sure what you mean by "check if the checksums on all mirror disks match". What checksums? checksums of all data blocks? That is as much work as a complete resync checksums of super blocks? That wouldn't tell us anything useful. NeilBrown - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
