How many disk should a home server have (I'm sure that was a song).

My previous post failed to answer all the questions.

Specifically how many disks should a home server contain?

Now I will gloss over the obvious answer of zero, all your data should be on the net managed by a professional organisation, not least as I would not trust someone else with my photos however good they claim to be. Also any self respecting geek will have a server at home with storage, which at the moment means spinning rust.

Clearly you need more than one disk for redundancy and you have already worked out that the only sensible choice for a file system is ZFS, you really don't want to loose your data to corruption. It is also reasonable to assume that this system will have 6 drives or less. At the time of writing you can get a Seagate 750Gb SATA drive for £151.56 including VAT or a 320Gb for £43.99.

Here is the table showing the number of disks that can fail before you suffer data loss:

Number of disks

Mirror

Mirror with one hot spare

RaidZ

RaidZ with hot spare

Raidz2

Raidz2 with one hot spare

2

1

N/A

N/A

N/A

N/A

N/A

3

NA

2\*

1

N/A

N/A

N/A

4

1\*\*

N/A

1

2\*

2

N/A

5

NA

2\*

1

2\*

2

3

6

1\*\*

N/A

1

2\*

2

3

\* To not suffer data loss the second drive to fail has to not fail while the hot spare is being re silvered.

\*\* This is a worst case of both disks that form the mirror failing. It is possible that you could loose more than one drive and maintain the data.

Richard has some more numbers about mean time before data loss and the performance of various configurations from a more commercial point of view, including a 3 way mirror.

Now lets look at how much storage you get:

Number of disks of size X Gb

Mirror

Mirror with one hot spare

RaidZ

RaidZ with hot spare

Raidz2

Raidz2 with one hot spare

2

X

N/A

N/A

N/A

N/A

N/A

3

N/A

X

2X

N/A

N/A

N/A

4

2X

N/A

3X

2X

2X

N/A

5

N/A

2X

4X

3X

3X

2X

6

3X

N/A

5X

4X

4X

3X

The power consumption will be pretty much proportional to the number of drives, as will the noise and cost of purchase. For the Seagate drives I looked at the power consumption of the disks was identical for the 300Gb and 750Gb drives.

Since my data set would easily fit in a 320Gb disk (at the time of purchase) and that was the most economic at that point I chose the 2 way mirror. Also raidz2 was not available.

If I needed the space offered by 2X or more disks I would choose the RaidZ2 as that gives the best redundancy.

So the answer to the question is “it depends” but I hope the above will help you to understand your choices.

Tags:

Comments:

problem is, disks don't go down as other hardware does in price :/ I built a small zfs home server in september 2006 with 3x320gb raidz pool, 88euro each, a 500gb drive at that time costed MORE than TWICE and didn't hold twice the data, the server works flawlessy, but now is full :( And what do I discover? that the "sweet spot" is still the 320gb disk! The 750 and 1tb which would be the next logical step (because 500 isn't even double the capacity so I'd like to jump it) are SO expensive :/ So I think that the only option for me could be to add another 3x320gb, zfs can do just that and this is awesome, but still, I'd really prefer to gracefully "swap" my 3x320gb disks with 3x1tb ones :'D

Posted by sickness on May 03, 2007 at 01:36 PM BST #

I'm not sure I agree about the price. A 320G drive is now almost 60% of the price I paid 9 months ago.

However I take your point that it would be nice to jump to 750GB or 1TB drives when I want to upgrade to more drives I will need a new, bigger case as this one only holds 3 and without a major dump and restore to external drives I can't move from a mirror to RAIDZ even if I wanted to.

ZFS is however fulfilling the original I in RAID, Redundant Array of Inexpensive Disks, without sacrificing data integrity.

Posted by Chris Gerhard on May 03, 2007 at 01:57 PM BST #

Are you setting up your SATA mirroring by using the BIOS RAID feature (nvidia MediaShield) ? I've got the same hardware and Solaris can't see my array at all during installation...

Posted by Geoff S on May 26, 2007 at 05:04 AM BST #

When I have ZFS!!! Hell no. I am not trusting my data to some bit of hardware that could be corrupting my data or hiding faults that come from the disk drives when I have the best file system on the planet checksumming every block.

See here and is configured like this

Posted by Chris Gerhard on May 26, 2007 at 11:55 AM BST #

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