[Scottish] Hardware question - slightly linux related :)

Colin McKinnon colin.mckinnon at ntlworld.com
Wed Jun 13 22:30:29 BST 2007

On Friday 01 June 2007 11:46, Phillip Bennett wrote:
> I am looking at replacing one of my Linux servers here at the moment and
> wanted to ask people about their experiences with hardware configuration..
> Basically, it will have about a terrabyte or so of space available in the
> form of a RAID array, (probably RAID5) 

Phillip - you've got this kind of backwards. A mopre sensible way round would 
be to start with the application, then price, then start thinking about 
physical arrangements and RAID levels.

> and I was wondering if anyone would 
> be able to tell me the pros/cons of having all this available in just ONE
> array, as opposed to having two or more smaller arrays.  

It really rather depends how you split the arrays; we don't know if RAID 5 is 
the right way to go for your data. Assuming it is, then it's so small it'd be 
difficult to do it in anything other than a single 'big' array. Depending on 
your budget you may want to eliminate the controller as a single point of 
failure - you probably don't want to put more than about 4 disks per channel 
anyway for bandwidth purposes.

> Are there 
> performance advantages that people know about?  I realiase that with two
> smaller arrays, there is less risk to data, but more cost for disks, but
> could this be offset by buying larger (750G) drives?

? this is nonsensical. Sure, if you layer say mirroring on top of striping, 
raid 5 on top of mirroring, you can get large volumes with higher 
reliability, but for a 1Tb array? The more disks you split your data over, 
the more reliable it should be but it doesn't follow it will be faster - 
bigger disks tend to be newer and faster.

> For those wondering, it'll be a PowerEdge 2950 with all disks internal, as
> opposed to an external array.  Will this make a difference?  

Currently there's not much to choose between SATA and SCSI for streaming reads 
but random reads / writes are still (IMHO) handled better by the latter. OTOH 
there's a huge price difference.

If you're wanting something substantial to go on, then, if its a fileserver, 
probably 5 250Gb SATA drives is the sensible way to go RAID5 across 4 of the 
disks with one as a hot standby. For a relational, transactional database, 
then RAID 1 (software) on top of RAID 0 (software or hardware) with SCSI 
disks is probably the sensible way to go.

I've just had a Google - the 2950 is kind of small (2U) - and you want all the 
disks internal???? Putting the disks internally is the only design constraint 
you've given - but it is a big problem! Try to rethink this if you can.


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