[linux-lvm] understanding snapshot

[linux-lvm] understanding snapshot

[Rich Turner]

i am trying to get a thorough understanding of the snapshot process. i
have searched around for documentation but was unable to find detailed
information. perhaps someone can point me in the right direction or tell
me if my assumptions are correct.

i understand my snapshot lv only needs to be 15-20% the size of my
original lv. that leads me to believe:

1. we only write to the snapshot lv when data from the original lv has
changed. this copies the original version of the file to the snapshot lv
so that when we backup the snapshot lv it uses the original file and not
the changed version. all files that were not changed will be taken from
the original lv. this process requires 2 writes and 1 read.

2. if there are numerous writes to the original lv after the snapshot
has been created (and before it has been removed) we could exceed our
snapshot lv size. this is because we have to copy the original versions
of the file to the snapshot lv, and depending on the number of writes
and the size of the files we could run out of logical extents. when this
happens the snapshot is unusable.

what will happen to the backup process (tar for example) of a snapshot
lv when it runs out of storage?

do chunks contain the metadata for the data in the original lv?

what is the effect of increasing/reducing the chunk size?

rich turner

Re: [linux-lvm] understanding snapshot

[Dale J. Stephenson]

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@mac.com