Backup of a live filesystem revisited

[#Bill.Stewart]

In article <4760002 at hpirs.HP> lkc at hpirs.HP (Lee Casuto) writes:
>A few weeks ago I posted a request for information about a paper on
>the backup of live filesystems. I also received lots of mail from
>.....
>>I hate to disappoint you, but no such paper exists. In fact I have gone
>>out of my way over the years to make it as clear as possible that `live'
>>dumps are NOT always going to work! The problem is that they usually do
>>work, particularly if they are being used to extract individual files.
>>But full incremental restores are likely to bomb out, and of course these
>>are the ones that are most critical.
>>	Kirk McKusick

There are two basic approaches to backups:
	programs that use the file system (e.g. tar, cpio), and
	programs that scrounge directly off the disk (dd, dump, volcopy, finc)

File-system based programs can work on live systems as long as the individual
files are not changing.  They are slow but flexible, and do incremental dumps
well.  Unfortunately, they can't tell when a given file has been *removed*, and
can get horribly confused if you play games with links or modification times
between dumps.

Disk-based backup programs are normally much faster, but are unsafe on live
file systems; if nothing's being written at 3AM you may luck out.
Disk-based *restore* programs are another story; you should expect terrible
corruption if you use one on a live disk.  Suppose someone was already using
inode 443 when you try to restore it?
-- 
# Bill Stewart, AT&T Bell Labs 2G-202, Holmdel NJ 1-201-949-0705 ihnp4!ho95c!wcs