git-annex/doc/copies.mdwn

The WORM and SHA1 key-value [[backends]] store data inside
your git repository's `.git` directory, not in some external data store.

It's important that data not get lost by an ill-considered `git annex drop`
command.  So, then using those backends, git-annex can be configured to try
to keep N copies of a file's content available across all repositories. 
(Although [[untrusted_repositories|trust]] don't count toward this total.)

By default, N is 1; it is configured by annex.numcopies. This default
can be overridden on a per-file-type basis by the annex.numcopies
setting in `.gitattributes` files. The --numcopies switch allows
temporarily using a different value.

`git annex drop` attempts to check with other git remotes, to check that N
copies of the file exist. If enough repositories cannot be verified to have
it, it will retain the file content to avoid data loss. Note that
[[trusted_repositories|trust]] are not explicitly checked.

For example, consider three repositories: Server, Laptop, and USB. Both Server
and USB have a copy of a file, and N=1. If on Laptop, you `git annex get
$file`, this will transfer it from either Server or USB (depending on which
is available), and there are now 3 copies of the file.

Suppose you want to free up space on Laptop again, and you `git annex drop` the file
there. If USB is connected, or Server can be contacted, git-annex can check
that it still has a copy of the file, and the content is removed from
Laptop. But if USB is currently disconnected, and Server also cannot be
contacted, it can't verify that it is safe to drop the file, and will
refuse to do so.

With N=2, in order to drop the file content from Laptop, it would need access
to both USB and Server.

Note that different repositories can be configured with different values of
N. So just because Laptop has N=2, this does not prevent the number of
copies falling to 1, when USB and Server have N=1. To avoid this,
configure it in `.gitattributes`, which is shared between repositories
using git.