Looking For RAID Hardware/Software Advice

[Chuck Sharp]

Mark, RAID 5 takes at least 3 disks. The reason for that is because of
the way stores data. RAID5 divides up the data it writes into groups.
For each groups of data being written, it will spread the data across
all but one disk. On the remaining disk, it will write the data's
parity calculations. This set of data can be used to re-create the
data in that group on any of the other drives that fails, by using the
remaining data in the group. Different sets of data written to raid5
will designate different disks for the parity data. Every single write
to a raid 5 involves some hefty calculations.

Raid1 is mirroring only. In other words, any data written to one disk
will be written to the other.

RAID10 takes 4 disks, because you are both mirroring and striping
data. I don't think that's what you really had in mind here.

Here's the upshot.
1. Writing is slower to a RAID5 set.
2. Writing to a software RAID5 set is often CPU intensive too.
3. RAID1 is much simpler technically and conceptually than RAID5.
4. RAID1 is a little simpler to set up.
5. RAID1 allows each volume to be used individually (you can "break
the mirror) without hiccups. RAID5 ties all the disks intimately
together, so you can't do that.
6. If a software RAID5 disk dies, it is CPU intensive to rebuild that
disk. Rebuilding a RAID1 disk is a simple block-by-block copy from one
to the other.

Generally, people opt for RAID1 or RAID10 configurations these days
(generally, there are plenty of exceptions) for those reasons. RAID5
was much more popular when storage was so expensive, but nowadays the
cost is so low that RAID1 is usually the way to go.

Chuck

On Mon, Jan 19, 2009 at 7:30 AM, Mark Phillips
<mark at phillipsmarketing.biz> wrote:
> Eric
>
> Thanks for the summary, and thank-you to everyone for their ideas.
>
> Based on NewEgg prices, here is some more information:
>
> Option A
> Single Disk IDE Drive - 500 GB and backups, keep OS on existing drive =
> $69.99
> Use existing controller and just add another drive. No redundancy
>
> Option B
> RAID10 with 500 GB backup capacity and redundancy, keep OS on existing drive
> = $179.97
> 2 500 GB SATA2 Drives, new SATA2 controller
>
> Option C
> RAID10 with 750 GB backup capacity and redundancy, keep OS on existing drive
> = $239.97
> Two 750 GB SATA2 Drives, new SATA2 controller
>
> Option D
> RAID5 with 1,000 GB backup capacity and redundancy, keep OS on existing
> drive = $239.97
> Three 500 GB SATA2 Drives, new SATA2 controller
>
> I am leaning towards Option C based on less power consumption with fewer
> drives. However, I have to rethink my budget...
>
> After some more reading, I am a little confused about the debate between
> RAID5 and RIAD10. I am interested in the group's opinions on which is better
> - RAID 5 or RAID 10 and why? What experiences have you had regarding
> installation, maintenance, and fixing problems? I am running Debian testing.
>
> Thanks!
>
> Mark
>
> On Sat, Jan 17, 2009 at 6:35 AM, Eric Shubert <ejs at shubes.net> wrote:
>>
>> Mark Phillips wrote:
>> > I am running out of room for my backups. I use backuppc and I have
>> > almost filled a 150GB drive with backups from 7 computers, and I need to
>> > add another 2 computers to the set. I have an old Dell Poweredge 1300
>> > server (Pentium III 550 Mhz, 500 MB RAM, PCI 33.3Mhz) that I could turn
>> > into a backup server. I am looking for suggestions/thoughts on how to
>> > set this up. I need to keep the cost down as much as possible; under
>> > $150.
>> >
>> > My initial thoughts:
>> >
>> > * Keep current 72 GB drive for OS (debian testing, about 68% full)
>> > * Add two 500 GB SATA drives and a PCI SATA controller ~$130
>> > * Software RAID and LVM for the two drives
>> > * Move current 150 GB of backups to the RAID
>> > * Backuppc now runs on this machine and slowly fills up the RAID
>> >
>> > My questions:
>> >
>> > 1. Should I keep the 72 GB drive for OS, or put it on the RAID?
>> >
>> > 2. I can add another CPU (P III 550 MHz) processor to the box - is it
>> > worth the effort to find one? I found one source for $5/CPU, I just need
>> > to find the heat sink and mounting hardware. Will this improve
>> > performance?
>> >
>> > 3. The box has a built-in SCSI 68-pin Ultra2/wide bus/controller, but
>> > SCSI drives are more expensive, at least from a cursory google search.
>> > Is this correct? I don't think I can use SCSI drives within my budget
>> > constraint.
>> >
>> > 4. Would upgrading the memory to 1GB improve performance - top shows:
>> > Mem: 646676k total,      639300k used     7376k free,      64548k
>> > buffers
>> > This would add another ~$60 to my cost.
>> >
>> > 5. Should I look at hardware RAID cards - they seem very cheap, so
>> > perhaps software is better?
>> >
>> > 4. Does this plan make sense, or is there a better way to proceed for
>> > about the same cost?
>> >
>> > Thanks!
>> >
>> > Mark
>> >
>>
>> Good replies, all. To sum things up, I think a SATAII PCI card (2 or 4
>> port) and 2 drives is all the HW you need to add to the backup box you
>> currently have. Set up the drives with SW RAID-1 (mirrored) and you're
>> good to go. Migrate the data to the raid device, and keep the OS on the
>> existing drive.
>>
>> With KeepItSimpleStupid in mind, I recommend using RAID-1 as opposed to
>> RAID-5. With the price of drives these days, the additional space you
>> get with RAID-5 isn't worth the headache you'll get when there's a
>> problem. With RAID-1, each drive can be mounted (and used) individually
>> if necessary. Not so with RAID-5.
>>
>> --
>> -Eric 'shubes'
>>
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