On Jun 11, 2003 16:30 +0200, Eric.Chacron@alcatel.fr wrote: > Andreas Dilger wrote: > >As for performance effects, that depends on how long you have your > >snapshot active, how much you write into the filesystem, and whether the > >snapshot is on the same or a different disk/controller. With any LVM > >snapshot, you need to copy the data from the original disk to the snapshot > >volume before you write new data there. If your original filesystem has > >a lot of writes, then you will now have 2x that many writes (1 going to > >the snapshot). Once data has been copied to the snapshot, however, it > >does not need to be copied again so you only need to copy the original > >superblock, inode tables, etc to the snapshot once. > > 2x many writes: i'm not aware of how LVM performs its copy on write > mechanism: > what's the data unit used for replication from the original LV to the > snapshot, when a page / block is modified on the original LV ? That is a function of how you created the snapshot LV. By default the chunk size is 64kB, but it can be given by "lvcreate -c <size>". Having a larger chunk size makes for more efficient storage of chunk metadata, but also more empty space in each chunk if you aren't modifying the whole chunk. Cheers, Andreas -- Andreas Dilger http://sourceforge.net/projects/ext2resize/ http://www-mddsp.enel.ucalgary.ca/People/adilger/ _______________________________________________ Ext3-users@redhat.com https://www.redhat.com/mailman/listinfo/ext3-users
