Re: LVM snapshots

On Mon, Mar 17, 2003 at 03:35:53PM -0000, Matt Clark wrote:
> > LVM-blocksizes are relatively large, 4 MB pr default, and for large
> > volumes it's common to use even bigger blocks (say 128 MB). For a 2TB
> > volume and 128 MB blocks the datastructure should take 32KB memory.
>
> 4MB is the size of a physical extent, not a block.  Snapshots use 'chunks' not whole extents, and I don't know what
(ifany) 
> relationship there is between chunks and blocks.
>
> #define LVM_SNAPSHOT_MAX_CHUNK  1024    /* 1024 KB */
> #define LVM_SNAPSHOT_DEF_CHUNK  64      /* 64  KB */
> #define LVM_SNAPSHOT_MIN_CHUNK  (PAGE_SIZE/1024)        /* 4 or 8 KB */

Ah, that explains it.
Sorry for adding to the confusion.

> Anyway, you definitely *don't* force LVM to copy 4MB of data the first time you touch just 1 bit on  a snapshotted
volume!

Probably not.


--
Ragnar Kjørstad
Zet.no