backup the entire server

[Aleksandar Milivojevic]

Claude Jones wrote:

> Contrary to the subject line, BackupPC doesn't NOT back up the entire server, 
> at least in WindowsLand. All files that are locked, which include things like 
> your Outlook .pst file, and many operating system files, will NOT be backed 
> up. I am too new to Linux to know how this issue plays out in the Linux 
> world.

I Unix world, there are no mandatory file locks, only advisory locks. 
If application chooses to ignore the lock, nothing is preventing it from 
accessing the file (including removing the file that some other process 
keeps open).

So, backup application under Unix *can* backup the locked file, if it 
chooses to ignore the locks (or doesn't check for them).  However, if 
the file was being kept open by some other application for writing, the 
content of the file might be corrupted/unusable after restore is 
performed (not to be confused with corrupted buckup or file, the backup 
and file will be OK, only the content might get corrupted).

There are couple of solutions to the problem:

  - stop applications that keep file open, then do the backup

  - simlar to above, if under LVM, make snapshot (instant operation), 
restart applications, do backup of snapshot, delete snapshot

  - similar to previous (but redundancy lost while backup runs), if on 
software RAID1, flush dirty pages to disk, detach one submirror, do the 
backup, than reattach the mirror (Solaris has a nice feature that Linux 
lacks, Solaris allows submirror to be offlined, instead of detached, in 
which case no resync of entire mirror is needed, only changed blocks are 
synced, plus handy lockfs command that prevents new dirty pages to be 
created between sync and offline (or detach) operations, so you are 
guaranteed to have correct data on offlined (or detached) submirror).

  - use application's backup module to do safe backups.  most databases 
come with some kind of dump tool, some come with its own backup tools 
and/or modules for backup systems such as Legato Networker or Veritas 
Netbackup), and some can create snapshots.  safe to use on live database 
as long as database supports transactions and/or locking *and* 
applications that access database are using them correctly (in other 
words, they don't leave database in unconsistent state between two 
transactions or wile tables are unlocked).  if your database can't be 
brought down not even for milisecond, this might be the only option for 
making a backup of it.

-- 
Aleksandar Milivojevic <amilivojevic at pbl.ca>    Pollard Banknote Limited
Systems Administrator                           1499 Buffalo Place
Tel: (204) 474-2323 ext 276                     Winnipeg, MB  R3T 1L7