At 3:06 PM -0800 12/15/2008, darkey wrote:
>
>what errors make their way onto a RAID unit that a clone backup
>won't be vulnerable to?
If something goes wrong on your file system, the errors are reflected
into the RAID volumes immediately. The easiest example is if you
delete the wrong files. They're deleted off the RAID volumes also!
So as a backup, the RAID is useless.
>How is this different than a clone - and it's subsequent backups?
A clone is a file-level bootable backup of your system, normally kept
*disconnected*. It is updated only when you connect it to your
system and run the backup app. IOW, those files you accidentally
deleted, will still be on the clone!
> > Time Machine is a file-level backup mechanism that runs periodically.
>> It's new and it's not 100% reliable yet. That's why many folx don't
>> like it.
>
>How is *this* different than a clone - and it's subsequent backups?
Time Machine uses a proprietary format, with hooks into the OS, to
keep your TM backup up-to-date. It really wants the backup volume
kept online... so it is vulnerable to system failures.
>Is it true that you can NOT boot from a TM backup?
You have to use Apple's tool to create a bootable system from the TM
backup. Depending on the size of your boot volume, this can take
hours. A TM backup is NOT a hot bootable clone.
> > >When swapping drives weekly, what is the procedure to update the
>> >freshly swapped drive?
>>
>> Just use CCC or SuperDuper to update the drive. It will replace only
>> the changed files, so the update won't even take that long.
>
>So when re-connecting the "alternate" {external #2} drive, what type
>of backup do I select; ie, incremental, update, etc - or do i need to
>"reclone" the drive again; I absolutely want the ability to be able to
>boot from whatever external I have ... as you can see - my luck isn't
>so hot!
The terminology changes depending on which app you use, but for the
most part "update" and "incremental" are interchangeable.
Once you create the clone in the first place (aka "populate the
volume"), you just run updates / incrementals. They compare the
original to the clone then update the things that have changed.
Put CarbonCopyCloner on your Mac now, launch it, and look thru it.
Read the docs. As long as you don't hit that Clone button (which
subsequently will require your admin password anyway), it will do no
harm...
In CarbonCopyCloner, you select the whole source drive, then under
"Cloning options" you select "Incremental backup" then check *both*
"Delete items that don't exist on the source" and "Archive modified
and deleted items". Those two checked options together, are very
important. The first says to make the destination identical to the
source -- a fully bootable clone. The second tells CCC to move the
changed things aside, into a special folder. (Special folder is a
misnomer; it's just a folder at the top level of your backup volume
with a name that begins with "_CCC". CCC knows to ignore it on
subsequent backups.)
Each time you run the incremental with CCC, using those options, it
will add another special folder at the top level of your clone. This
is CCC's equivalent of the history Time Machine is trying to provide.
But unlike TM - that clone IS bootable and the history is directly
perusable in the Finder. Those folders contain the files you deleted
/ modified, even after you've made the rest of the clone idential to
your source! When you get around to it, once you know you don't need
the data, you can trash those special folders.
> > Note that there is no such fast update for RAID1. Because it is
>> block-level, it will have to re-write the whole drive every time you
>> reconnect it. Some RAID hardware is a smart enough to read both
>> drives and compare the data and only write what's needed... but
>> still, it requires reading EVERY block on both the drives...
>
>I see the GM unit from OWC will automatically update the other drive
>in the enclosure should one need to be replaced. Is that the same
>thing?
yes. Note also that while a RAID volume is being updated like that,
its i/o system will be very heavily used - so the whole thing will be
sluggish for hours.
>Beyond that, should i get a RAID1 unit, what is involved in getting
>the freshly swapped drive to accurately - and *SIMPLY* - back up my
>internal HDD?
IF that particular RAID unit supports hot swapping drives... and IF
the RAID is working properly... You plug the drive into the box, the
box automagically does a *complete* block-level copy onto the drive,
and hours and hours later you can just pull the drive.
heh. That sounds like it's making a backup, right? Noooo. It's
adding the drive to the RAID! As you make changes in your system
(ie, accidentally delete files) it's doing it on the RAID also!!!!!
AND while the drive is being populated the whole RAID system's
performance will be degraded.
Likewise, that your history of changes (those deleted files), that
would have been kept by CCC or Time Machine are NEVER kept by RAID
solutions.
>No matter how much I've read on this, I keep seeing the phrase "RAID
>is not a substitute for backup"; even though I'm a little confused by
>that statement, it *seems* to be beginning to make sense to me?!
RAID was invented to solve a problem in business environments, with servers:
1) Servers MUST be up and running 24/7.
2) Servers MUST have massive pools of storage, available 24/7.
3) Sometimes the performance of an individual drive isn't fast enough.
4) The failure of a hard drive in that pool CANNOT take down the pool
or the servers.
Notice that backups are NOT listed there. Backups are copies of your
data to be kept OFF LINE so the things that happen on your system
(intentional, accidental, or acts of god), do NOT destroy your data.
HTH,
- Dan.
--
- Psychoceramic Emeritus; South Jersey, USA, Earth
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