[linux-lvm] read-only media in a LVM?

Wolfgang Weisselberg uzx87lvfmukwc001 at sneakemail.com
Mon Jun 23 16:23:01 UTC 2003

Sean P. Kane wrote 36 lines:

> Does a snapshot create a copy of all the files in memory?

Nope.  And not in memory either.

> Just the files that changed?

Nope, LVM is filesystem-agnostic, and that would have to change
that.  More code, more errors, less usability.

Additionally, what would you do with metadata, like used
filesystem blocks and timestamps and filenames?

Or with a 1 TB file where just 1 bit was changed?

> Or just the file blocks or contents (i.e. diffs) that have
> changed?

Blocks of (default) 32Kb of raw disk.  They are written to the
snapshot.  However, the tables of which blocks were written are
kept in RAM, so a read request on the snapshot redirects only the
unchanged blocks to the original (HD/CD) and gets the changed
blocks out of the snapshot storage.

> So, would the snapshot have to be the same size as the cdrom or
> could it be smaller?

MUCH smaller. Unless you insist on changing something in every
32KB block of the CDrom.


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