What raid to build on that drives?

What raid to build on that drives?

[Timo Boettcher]

Hi,

   I have 3 * 36GB discs and 5 * 18GB discs on two u160 scsi channels?
   What possibilities do I have to build a raid on them? I want to be
   able to survive one failed disc, but get as much space and
   performance out of the discs as possible.
   
 Timo

Re: What raid to build on that drives?

[Dan Egli]

> Hi,
>
>    I have 3 * 36GB discs and 5 * 18GB discs on two u160 scsi channels?
>    What possibilities do I have to build a raid on them? I want to be
>    able to survive one failed disc, but get as much space and
>    performance out of the discs as possible.
>

Best I can suggest is a raid 5. You'll get 72GB of space, good performance
(roughly what you'd get from a sigle drive), and if one of the drives goes
south, the other two will keep your raid set up till the drive can be
replaced.

--- Dan

Re: What raid to build on that drives?

[Timo Boettcher]

Hi Dan,


* Dan Egli , Wednesday, November 26, 2003, 8:59:02 PM:

>> Hi,
>>
>>    I have 3 * 36GB discs and 5 * 18GB discs on two u160 scsi channels?
>>    What possibilities do I have to build a raid on them? I want to be
>>    able to survive one failed disc, but get as much space and
>>    performance out of the discs as possible.
>>

> Best I can suggest is a raid 5. You'll get 72GB of space, good performance
> (roughly what you'd get from a sigle drive), and if one of the drives goes
> south, the other two will keep your raid set up till the drive can be
> replaced.

I hoped to integrate all discs in that raid....
I tried to run raid0 over pairs of the 18GB discs (leavin the fifth
as spare) and run a raid 5 over these raid0's and the three 36gb
drives, but that gives me 50mb/s reading as opposed to 100mb/s with
8*18GB raid5 on the same machine (three of these discs failed, so I
got bigger ones). Why did I loose that much performance?


 Timo

Re: What raid to build on that drives?

[Neil Brown]

On Wednesday November 26, spida@gmx.net wrote:
> Hi Dan,
> 
> 
> * Dan Egli , Wednesday, November 26, 2003, 8:59:02 PM:
> 
> >> Hi,
> >>
> >>    I have 3 * 36GB discs and 5 * 18GB discs on two u160 scsi channels?
> >>    What possibilities do I have to build a raid on them? I want to be
> >>    able to survive one failed disc, but get as much space and
> >>    performance out of the discs as possible.
> >>
> 
> > Best I can suggest is a raid 5. You'll get 72GB of space, good performance
> > (roughly what you'd get from a sigle drive), and if one of the drives goes
> > south, the other two will keep your raid set up till the drive can be
> > replaced.
> 
> I hoped to integrate all discs in that raid....
> I tried to run raid0 over pairs of the 18GB discs (leavin the fifth
> as spare) and run a raid 5 over these raid0's and the three 36gb
> drives, but that gives me 50mb/s reading as opposed to 100mb/s with
> 8*18GB raid5 on the same machine (three of these discs failed, so I
> got bigger ones). Why did I loose that much performance?
> 

It's hard to say...  It would depend in the chunksizes that you
used and the exact configuration.  Probably is just wasn't keeping all
the drives busy all the time.

The "best configuation" for you depends on things like:
  1/ do you want one large filessytem, or will two filesystems be ok?
  2/ If another 18GB drives dies and you replace it with a 36GB drive,
     will you be keen to incorporate that drive into the array?
  3/ How do you do backups? How big a filesystem can you dump/restore
     to allow for raid reconfiguration?

The only general rule I can give you is to *not* do raid5/raid1 over
raid0/linear.   Always do raid5/raid1 out of real drives, and then if
you want to, join those raids together with raid0/linear.  This gives
much better reliability in the case of failure.

I would probably partition the 36GB drives into 2 18GB partitions,
then use raid5 overall 8 drives (maybe 7 and a hot-spare if you want
that), and then raid1 with a hot spare for the 3 remaining "top-half"
18GB partitions.  Finally raid-linear the raid5 together with the
raid1.

NeilBrown

Re: What raid to build on that drives?

[David Rees]

On Wed, November 26, 2003 1at 1:36 am, Timo Boettcher wrote:
>
>    I have 3 * 36GB discs and 5 * 18GB discs on two u160 scsi channels?
>    What possibilities do I have to build a raid on them? I want to be
>    able to survive one failed disc, but get as much space and
>    performance out of the discs as possible.

How about two RAID5 arrays, one with the 3*36GB=72GB and the other using
the 5*18GB=72GB.  Then throw both of these under a RAID0 which will get
you 144GB and still be able to survive 1 disk failure.

-Dave

Re: What raid to build on that drives?

[Timo Boettcher]

Hi David,


* David Rees , Wednesday, November 26, 2003, 10:44:59 PM:

> On Wed, November 26, 2003 1at 1:36 am, Timo Boettcher wrote:
>>
>>    I have 3 * 36GB discs and 5 * 18GB discs on two u160 scsi channels?
>>    What possibilities do I have to build a raid on them? I want to be
>>    able to survive one failed disc, but get as much space and
>>    performance out of the discs as possible.

> How about two RAID5 arrays, one with the 3*36GB=72GB and the other using
> the 5*18GB=72GB.  Then throw both of these under a RAID0 which will get
> you 144GB and still be able to survive 1 disk failure.

that way i'll loose 54gb... but it seems reasonable...
i'll try it.


 Timo

RE: What raid to build on that drives?

[Bailey, Scott]

>>    I have 3 * 36GB discs and 5 * 18GB discs on two u160 scsi channels?
>>    What possibilities do I have to build a raid on them? I want to be
>>    able to survive one failed disc, but get as much space and
>>    performance out of the discs as possible.

> How about two RAID5 arrays, one with the 3*36GB=72GB and the other using
> the 5*18GB=72GB.  Then throw both of these under a RAID0 which will get
> you 144GB and still be able to survive 1 disk failure.

Here's a twisted thought, which would increase your available space, but
probably at the expense of some amount of performance degradation:

1. Partition your 36GB disks so first partition is the same size as one of
your 18GB disks, and the second partition is the remainder of the disk (also
about 18GB).

2. Create a RAID5 array containing 8*18GB disks (5 18GB disks + 3 18GB
partitions) = approx 126GB usable.

3. Create a RAID5 array containing 3*18GB partitions = approx 36 GB usable.

4. At this point, I would *NOT* recommend building a RAID0 array from (2)
and (3), as you will end up generating a lot of contention on the 36GB disks
by doing so. Either use them as separate devices, or concatenate them (using
LVM perhaps) if you really need all of the space in "one place".

This will get you a total of about 162GB usable while still being able to
survive a single disk failure.

I imagine simultaneous rebuilds of both RAID5 devices would not be a
pleasant experience. :-)

Cheers,

	Scott
	scott.bailey@eds.com

RE: What raid to build on that drives?

[Neil Brown]

On Monday December 1, scott.bailey@eds.com wrote:
> 
> 4. At this point, I would *NOT* recommend building a RAID0 array from (2)
> and (3), as you will end up generating a lot of contention on the 36GB disks
> by doing so. Either use them as separate devices, or concatenate them (using
> LVM perhaps) if you really need all of the space in "one place".

or md/linear.

> 
> I imagine simultaneous rebuilds of both RAID5 devices would not be a
> pleasant experience. :-)
> 

It is never nice to have to rebuild an array (though it is nice that
it just works), however this case is no worse than any other.  md will
notice that the two raid5 arrays share some physical drives and will
serialise the rebuilds.

NeilBrown