Norman> What you should be able to do with software raid1 is the Norman> following: Stop the raid, mount both underlying devices Norman> instead of the raid device, but of course READ ONLY. Both Norman> contain the complete data and filesystem, and in addition to Norman> that the md superblock at the end. Both should be identical Norman> copies of that. Thus, you do not have to resync Norman> afterwards. You then can backup the one disk while serving the Norman> web server from the other. When you are done, unmount, Norman> assemble the raid, mount it and go on. I tried both variants of Norman's suggestion on a test machine and they worked great. Shutting down and restarting md0 did not trigger a rebuild. Perfect! And I could mount component partitions read-only at any time. However on the production machine the component partitions refused to mount, claiming to be "already mounted". Despite the fact that the component drives do not show up anywhere in lsof or mtab. When I saw this, I got nervous and did not even try stopping md0 on the production machine. # mount -o ro /dev/sdc1 backup mount: /dev/sdc1 already mounted or backup busy The two machines hardly match. The test machine has a 2.4.27 kernel and JBOD drives hanging off a 3ware 7xxx controller. The production machine has a 2.6.12 kernel and Intel SATA controllers. Both machines have mdadm 1.9.0, and the discrepancy in behavior seems weird to me. Any insights? Paul> There have been a couple bug fixes in the bitmap stuff since Paul> 2.6.13 was released, but it's stable. You'll need mdadm 2.x as Paul> well. It turns out Debian has not yet packaged 2.6.13 even in the unstable branch. I will wait for this to happen before trying out the whizzy intent-logging and write-mostly suggestions. I'm brave, but not THAT brave. Dean> i didn't realise you were using reiserfs... i'd suggest Dean> disabling tail packing... but then i've never used reiser, and Dean> i've only ever seen reports of tail packing having serious Dean> performance impact. Done, thanks. Bill> If you want to try something "which used to work" see nbd, Bill> export 500GB from another machine, add the network block device Bill> to the mirror, let it sync, break the mirror. Haven't tried Bill> since 2.4.19 or so. Wow, nbd (network block device) sounds really useful. I wonder if it is a good way to provide more spindles to a hungry webserver. Plus they had a major release yesterday. While I've been focusing on managing disk contention, if there's an easy way to reduce it, that's definitely fair game. Some of the other suggestions I'm going to hold off on. For example, sendfile() doesn't really address the bottleneck of disk contention. I'm also not so anxious to switch filesystems. That's a two week endeavor that doesn't really address the contention issue. And it's also a little hard for me to imagine that someone is going to beat the pants off of reiserfs, especially since reiserfs was specifically designed to deal with lots of small files efficiently. Finally, I'm not going to focus on incremental backups if there's any prayer of getting a 500GB full backup in 3 hours. Full backups provide a LOT of warm fuzzies. Again, thank you all very much. -Jeff - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
