[linux-lvm] understanding snapshot

Dale J. Stephenson dalestephenson at mac.com
Thu Jan 15 23:52:01 UTC 2004

On Thursday, January 15, 2004, at 03:45  PM, Rich Turner wrote:
> what will happen to the backup process (tar for example) of a snapshot
> lv when it runs out of storage?
Reads from the snapshot will return I/O errors.  The backup process 
will likely fail.

> do chunks contain the metadata for the data in the original lv?
If metadata is written to the original lv, then yes.

> what is the effect of increasing/reducing the chunk size?
Depending on your write pattern, smaller chunk sizes may make the 
snapshot use space more slowly.  (If you write even 1K to a 4 MB chunk, 
the whole chunk will be copied.)  But it will also cause there to be 
more physical chunks to exist in a snapshot volume of the same size.  
Using LVM1 I ran into problems with snapshots eating up precious 
vmalloc memory, so I tend to use a large snapshot chunk size.  YMMV.

Dale J. Stephenson
dalestephenson at mac.com

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