Re: How's RAID doing?
From: Måns Rullgård (mru_at_inprovide.com)
Date: 06/12/05
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Date: Sun, 12 Jun 2005 16:27:44 +0200
martingerxt@yahoo.com writes:
> I'm wondering how people feel about RAID in various settings. My
> experience with it has not been great. The main reason I would want to
> use it is for redundancy, not speed improvements if there are in
> actuallity any speed improvements. My take is that redundancy and
> mirroring is more complex than RAID's monolithic data rudundancy
> scheme. Not all data is equal and you need to have a bit more
> intelligence in the scheme, for instance database replication schemes.
>
> Here's some points I would make about it. Like I said, I'm intreasted
> in data redundancy and backups, not speed.
First off, anyone doing anything more serious than gaming or web
browsing should use RAID.
> Questions:
>
> 1. Do people commonly use it on replicated database servers? It seems
> to me 2 or more single disk replicated servers would suffice for data
> integrity.
Replicating data across multiple servers will certainly preserve the
data. The advantage of using RAID here is that when a disk fails, the
task of repairing the server becomes much simpler: drop in a new disk
and wait.
> 2. Is the idea that you can easily put together an installation where
> all you do is swap out faulty hard drives periodically realistic?
That is how people normally use it, and with a high degree of success.
> My reading of various internet discussions is that RAID fails a
> lot.
You are more likely to read about failures than successes in the
newsgroups.
> It's common to blame faulty setups for the failures, but they in
> fact fail none-the-less. In my experience, the only server I've
> ever had that's lost data was a RAID machine.
Why did it lose data? How many machines have you run, and for how
long?
> 3. Aren't modern data integrity problems more complex than purely
> redundancy? For instance a worm attack can not be solved with RAID.
> It's better to keep a clean OS install on a single disk where backed up
> data can be copied onto or is already in place.
RAID is not a substitute for backups. RAID is very useful for
preventing damage caused by sudden catastrophic disk failures. It
does nothing against silent corruption of data by hardware or
software.
> RAID pros:
> 1. Potentially simple and reliable redundancy scheme.
>
> 2. Simple to maintain once set up. Don't have to worry about swapping
> out hard drives before they fail.
>
> 3. Potential for 100% up time.
That pretty much sums it up.
> RAID Cons:
>
> 1. More complex than single disk installs.
Sure, but not by much.
> 2. More expensive than single disk installs.
Also true. However, the benefit far outweighs the additional cost,
IMHO.
> 3. Not completely compatible with the idea of modern multi-node
> distributed server systems.
Those are totally unrelated. A node in a distributed system still
benefits from redundant disk drives.
> 4. Software/hardware is less widely tested and used than single disk
> IDE/ATA system making the systems inherently less reliable.
The drives are the same, so they are tested equally well. RAID
controllers are certainly less widely used, but still well on the side
of being adequately tested. Software RAID is also widely used, and
problematic setups are usually quickly identified. OTOH, it is a fact
that disks are inherently unreliable, and will fail, sooner or later.
> 5. When hard disks fail, they invariably give advance warning. This
> gives administrators time to replace it before it goes out. This is
> not always the case, but good backups and/or mirrors can handle this
> situation.
Hard disks can and do fail without so much as a whisper in advance.
Restoring from backups is also a cumbersome process, and backups will
never be fully up to date. The downtime incurred by a failure and
subsequent restore may also be highly undesirable. A RAID system,
especially with hot spares and hot-swappable drives, can have 100%
uptime, even when disks fail. Using RAID also makes the maintenance
after a failure less urgent, reducing the need for having staff
available at all times.
In my view, the advantages are well worth the somewhat higher hardware
costs.
-- Måns Rullgård mru@inprovide.com
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