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Re: how to use LVM snapshot with ext3 - VFS lock patch applicability



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/




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