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RAID1 - mirroring for a LAMP application

Posted on 2006-05-11
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I'd like to get a new server set up for a LAMP application.

    L=Linux, A=Apache, M=PostgreSQL (!), P=PHP

The database isn't ever going to exceed 1GB. It needs to be fast, but each transaction is valuable, so I'm thinking of using 2 hot-plug mirrored disks (RAID1) as well as the nightly backup to a remote server.

Here are my questions:

(1) Is the superior performance of a 15,000 RPM HDD likely to be noticeable over a 10,000 RPM HDD, bearing in mind my data set is not all that large?
(2) What is a sensible choice for a RAID controller for my configuration (64MB cache / 128 MB cache, why would I need an external channel?)
(3) What if... Catastrophe strikes and one of by HDD fails.
  (a) Does business continue as usual with no intervention?
  (b) How do I know one HDD has failed?
  (c) What steps are required to replace the failed HDD?

Apologies for the multi-part question. I'm expecting to split points if experts only want to answer parts.
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Question by:rstaveley
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rindi earned 1800 total points
ID: 16656915
1. Yes
2. The more cache, the better. Very good products are made by http://3ware.com/
3. Raid1 = Disks are redundant (mirror), one fails, the other still runs as normal.
a. yes
b. Depends on the controller and software. Some have utilities that will tell you of the status. It should also be recorded in the logs.
c. Also depends on the hardware. With some you just replace the disk, even if the system is running. If you are using an online spare (3rd disk), that will kick in and take over, after the mirror is rebuilt to the spare, everything works as normal.

If you need more speed, consider raid10 (2 mirrors spanned into one HD, total 4 disks).

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by:rstaveley
ID: 16657072
Thanks for the swift response, rindi.

> 3c. ...after the mirror is rebuilt to the spare..

Is that generally something the RAID controller takes care of automatically as soon as you plug in the new disk, or is there a procedure to follow?

> If you need more speed, consider raid10 (2 mirrors spanned into one HD, total 4 disks).

The existing sytem is has a single 1.4MHz processor with 32K cache (bogomips 2778.83) - i.e. an "entry level" server ~4/5 years ago - and the new system will probably have twin dual core Xeon 2.8MHz processors with 2M cache (bogomips 5605.36). It is conceivable that the existing application is CPU-bound, and the improved processing power is what we need.

Is there an easy way to profile an existing Linux application to see if disk I/O is the bottle-neck. The application is currently live on a system with no striping (or mirroring). How would I be able to find out if the disk I/O warranted striping? RAID10 would require a larger chassis and significantly increase the cost of the sytem, but if there was a significant performance improvement because if it (e.g. a >10% faster application response times), it is worth considering.
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by:rindi
ID: 16657169
If you have a builtin spare disk (3 disks) then as soon as the array breaks, the spare is automatically activated by the controller. If you don't have a spare builtin, then you need to remove the bad disk and replace it first, and then most controllers will take over, but not all. Raid 10 can be added later if the speed isn't enough.
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by:rstaveley
ID: 16657301
> Raid 10 can be added later if the speed isn't enough.

So I guess I'd need >= 5 hot-pluggable drive bays to cover my options. I'd start with 2 disks (or 3 to get the built in spare) for RAID1 and scale up to 4 (or 5 for a built in spare, if I was suffering from slow disk I/O.

Any idea how I'd know if there would be siginficant benefit from striping... or do you reckon that's a question for another TA?
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by:rindi
ID: 16657356
It's difficult to say just like that, but as a general example, both writes and reads would happen to 2 disks at the same time, appart from the mirroring, so the speed would be almost double (less some overhead).
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by:rstaveley
ID: 16658538
Thanks for all the help, rindi
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Expert Comment

by:rindi
ID: 16659109
your welcome.
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