Commit df26ed216f09d9df8d94cbdd6bfc591d1f6a7fa2 - Space-Madness

Define 'backed up' for depositories, and talk a bit about repos. Chris Pressey 6 years ago

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19	19	A subject is expected to have the same content in all the different depositories
20	20	in which it appears. (This is not 100% true, but it's useful to think of them
21	21	this way.)
22
23		Because subjects are hierarchical, it is also possible to think of (and treat)
24		a subject as a sort of sub-depository.
25	22
26	23	There are three kinds of depositories:
27	24

54	51	(perhaps because the filesystem on which the canonical depository is stored
55	52	is not available at the moment) but which are intended to be put there soon.
56	53
57		There are two important actions on depositories.
	54	With these definitions, we can say that a canonical depository is _backed up_
	55	if and only if every subject in it exists in at least one cache depository
	56	which resides on a different physical medium than the canonical depository.
	57
	58	Note that two partitions on the same hard disk are not different physical
	59	media!
	60
	61	The goal of the backup system is that every interesting file is in a canonical
	62	depository, and that every canonical depository is backed up.
	63
	64	To accomplish this goal, there are two important actions on depositories.
58	65
59	66	### Update a cache depository from a canonical depository ###
60	67

80	87
81	88	but incorporates some consistency checking, i.e. that you don't trample
82	89	something in the canonical depository with something in the incoming with the
83		same name.
	90	same name. (Also, `rsync` is pretty picky about directory names, and if
	91	you forget the trailing slash, bad things can happen. So it helps there too.)
84	92
85	93	The `deposit` tool takes an additional flag, `--clean`, which deletes all the
86	94	files (but not the directories) in the incoming depository after they are

89	97
90	98	It is an excellent idea to update a cache depository from the canonical
91	99	depository after running `deposit` (and before running `deposit --clean`.)
	100
	101	### Advanced Topics in Backup Subjects ###
	102
	103	Because subjects are hierarchical, it is also possible to think of (and treat)
	104	a subject as a sort of sub-depository.
	105
	106	It is up to you to decide if you have any backup use cases that are complex
	107	enough to justify doing that.
	108
	109	It is also useful to keep certain backups in version-controlled repositories,
	110	e.g. git repos. In this case, a repo directory can usually be treated as
	111	a subject. Instead of depositing files with `deposit`, one can push changes
	112	to the repo-subject residing in the canonical depository.
	113
	114	### Read-only Depositories ###
	115
	116	Need to research this, but the basic idea would be to `chmod -R` a
	117	cache depository so that you don't accidentally change files in it
	118	or add files to it. (But then changing the permissions back when you
	119	need to update it from a canonical depository.)
92	120
93	121	Additional tools
94	122	----------------