On Jan 24, 2003 22:07 +0900, Sean Oh wrote: > I am having difficult with LVM snapshots and syncing. > I have tested LVM 1.0.6 and also LVM2. Both generate the same problem. > > My testing environment is as follows: > > VG vg01 Size 100GB > LV lv01 Size 50GB > LV lv01_snapshot1 size 10GB > LV lv01_snapshot2 size 10GB > LV lv01_snapshot3 size 10GB > > as you can see above, there 4 LVs, one LV and 3 snapshots for the LV > and also the system has SCSI raid controller(Mylex) and 1024GB of RAM, > though these does not seem to matter, since the problem happens to other > systems, also. > The FS does not matter here, since Reiserfs and XFS both produce the same > syncing problem. > > After copying a file with 1GB size to lv01 and typing 'sync' takes more 30 > minitus or sometime forever. During these time period, access to lv01 is > neary blocked, producing very slow performance(0 ~ 400B/s). If I remove the > snapshots, sync returns normally. So it is the issue with LVM making the > changes in LV to 3 snapshots while locking the LV. The problem is that when you type "sync" you are actually writing (most of) your file to disk 4 times - once for the initial LV (which was mostly still in RAM I'm guessing), and once for each snapshot. Since you now have 4x the write load and it is not streaming, but random I/O it causes your disks to thrash badly. Add to that the fact that you are writing to a RAID which needs to seek _every_ disk for each write, and you are in really bad shape. I'm guessing that if you put each snapshot on its own physical disk (maybe only keeping the original LV on the RAID device) you will see a lot of improvement. Also, it would be interesting to know what the real performance of even the single 1GB write is (i.e. write + sync) because I'm guessing most of the data is still in memory. At, say, 50MB/s to disk you are still talking 3 or 4 minutes to write out 1GB, so even an ideal LV snapshot case would take 9-15 minutes to write out 4GB. Cheers, Andreas -- Andreas Dilger http://sourceforge.net/projects/ext2resize/ http://www-mddsp.enel.ucalgary.ca/People/adilger/ _______________________________________________ linux-lvm mailing list linux-lvm@sistina.com http://lists.sistina.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
