Re: What is RAID0 good for?

David Brown <david@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On 16/08/2010 14:52, Arno wrote:
David Brown<david@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On 16/08/2010 05:47, Arno wrote:
GMAN<Winniethepooh@xxxxxxxxxxxxxxxx> wrote:
In article<8b469fff-e694-4283-83cb-dc1db4ecc379@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, "mscotgrove@xxxxxxx"<mscotgrove@xxxxxxx> wrote:
On Aug 14, 10:44=A0am, M.L.<m...@xxxxxxxxxxxxxxx> wrote:
My friend has a Win XP =A0PowerSpec B647 desktop with two 250 GB hard
drives configured in the BIOS as RAID0. It appears that whatever
happens to one drive happens to the other, so when she was infected
with a virus both hard drives were infected, and both were
simultaneously cleaned by the same antivirus scanner.

What good is having those 2 drives perform like that when she is a
non-techie home user who would be better off with the use of 2
independent hard drives? Is there a way to turn off the RAID behavior
so she can get usage more suitable to her needs? Thanks.

RAID 0 is nor really RAID as there is no redundancy. As discovered,
if one drive fails, in effect all data is lost.

The reason for it, in theory it will make the drive run faster. For
the average user, this speed difference may never be seen.

A safer option would be to use JBOD (Just a Bunch Of Disks) and
concatinate the two to make a single 500GB drive.


How is JBOD "safer" than Raid0 ?

It is, but only by a tiny bit and only with the right
filesystem. The thing is that with RAID0, all data is lost
on a disk failure. With concatenated disks, you may recover
data from the non-failed disk.


"JBOD" has two meanings - it can mean "spanning" or "concatenation",
which is what you mean here, or it can mean "treat as several
independent disks".

Actually it typically means either "spanning" or "independent disks
in one storage device". It makes absolutely no sense to call some
independent disks a "JBOD". Not that people have been using that
term for a lot of things. It actually has no defined meaning at all.


I don't claim Wikipedia to be the "definition", but it does show the usage:

<http://en.wikipedia.org/wiki/JBOD#JBOD>

I think "independent disks in one storage device" and "independent
disks" really mean the same thing - each disk is treated by itself.
Anyway, it's not worth arguing about - we both know what I mean, and we
agree there is no fixed definition of the term.

I can agree to that ;-)


For spanned disk sets, you have little improvement in safety over Raid0
- files are often scattered around so that you will often have parts on
each disk, and metadata in particular is often spread over the disk
(unless the disk space is very underused - in which case a single disk
is a better choice). In particular, if the first disk is lost then you
will practically speaking lose everything.

That is why I said "very little", as in "insignificant".


Fair enough.

Treating the two disks entirely independently is a lot safer, especially
if you copy data across the two disks regularly. So that sort of "JBOD"
is much better than Raid0 (and much safer than Raid1 for typical home
usage).

Both options are strictly for temporary, low-reliability
storage. Don't use them to hold production data that is
not replicated in other places.


Raid is not about data safety - it's about uptime/downtime, and the
convenience of not having to restore from backup when a disk dies. It
doesn't replace backups and data replication.

I don't agree. RAID (nonzero) does replace replication, it does not
replace backup. It happens to be about data safety, it just
gives you a worse coverage than a backup, so you typically still
need a backup in addition. But if you, for example, do not need
a backup but can do a new installation with fixed, well known
effort, RAID is one way to bring teh failure probability down
enough that a backup becomes cost-ineffective.


If you can get everything back from a re-installation, then your
installation source is your backup.

Hmm. Ok, that _is_ a legitimate view.

RAID gives you redundancy on the hard disk hardware - that's all. It
reduces the risk of losing time or data due to a hardware failure, but
does nothing towards reducing the risk from other conditions (malware,
user error, or file system corruption). You can argue the benefits of
"replication", "backup" and "data safety" as much as you want but in
reality it comes down to what can go wrong, the chances of that
happening, and the consequences of such failures. Raid covers a certain
class of failure (hard drive failure) which is typically fairly low
risk, and reduces the consequences to almost nothing. That's definitely
nice.

Well, the risk of disk failure has certainly gone down. I lost
several disks and went to RAID some years ago. Of course none
of my disks has failed permanently since.

Incidentially, RAID also covers things that Backup does not, namely
non-permanent disk failure and uptime improvement.

Concrete example: I have a low-power fileserver/firewall/email-
server with 3-way RAID1 on notebook disks. Every few months,
one of the notebook disks has a problem and becomes inacessible.
Without the RAID, the server would crash. With it, it stays up,
and I pull and replug the disk when I have time. These failures
so far were all non-permanent, nothing in the SMART log, no data
corruption, nothing.


But since raid does not cover the other sources of failure that are
generally more common, it is at best a small part of a data safety
solution. And something that /does/ cover such failures - a good backup
solution - also covers hard drive failure. So the backup is your data
safety solution. Raid enhances that by reducing the consequences of
certain failures (no lost uptime, and no loses of data that has not yet
been backed up). But it is nothing more than a little add-on to a
backup solution, and certainly a waste for home usage like the O/P's.

Lets just say that RAID and backup cover mostly different problems.
In practically all cases "RAID is not backup!" does cover it and
what people usually need is backup.

However for some things, it is ceratinly important and there
"Backup is not RAID!" is even more true.

Arno
--
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@xxxxxxxxxxx
GnuPG: ID: 1E25338F FP: 0C30 5782 9D93 F785 E79C 0296 797F 6B50 1E25 338F
----
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans
.

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