hi,

this is not really postgresql specific, but any help is appreciated.
i have read more spindles the better it is for IO performance.

suppose i have 8 drives , should a stripe (raid0) be created on
2 mirrors (raid1) of 4 drives each OR should a stripe on 4 mirrors
of 2 drives each be created ?

also does single channel or dual channel controllers makes lot
of difference in raid10 performance ?

regds
mallah.

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Stripe of mirrors is preferred to mirror of stripes for the best balance of
protection and performance.

In the stripe of mirrors you can lose up to half of the disks and still be
operational. In the mirror of stripes, the most you could lose is two
drives. The performance of the two should be similar - perhaps the seek
performance would be different for high concurrent use in PG.

- Luke


On 5/29/07 2:14 PM, "Rajesh Kumar Mallah" <mallah.rajesh@gmail.com> wrote:

> hi,
>
> this is not really postgresql specific, but any help is appreciated.
> i have read more spindles the better it is for IO performance.
>
> suppose i have 8 drives , should a stripe (raid0) be created on
> 2 mirrors (raid1) of 4 drives each OR should a stripe on 4 mirrors
> of 2 drives each be created ?
>
> also does single channel or dual channel controllers makes lot
> of difference in raid10 performance ?
>
> regds
> mallah.
>
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>



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On 5/30/07, Luke Lonergan <llonergan@greenplum.com> wrote:
> Stripe of mirrors is preferred to mirror of stripes for the best balance of
> protection and performance.

nooo! i am not aksing raid10 vs raid01 . I am considering stripe of
mirrors only.
the question is how are more number of disks supposed to be
BEST utilized in terms of IO performance for

1. for adding more mirrored stripes OR
2. for adding more harddrives to the mirrors.

say i had 4 drives in raid10 format

D1 raid1 D2 --> MD0
D3 raid1 D4 --> MD1
MD0 raid0 MD1 --> MDF (final)

now i get 2 drives D5 and D6 the i got 2 options

1. create a new mirror
D5 raid1 D6 --> MD2
MD0 raid0 MD1 raid0 MD2 --> MDF final


OR

D1 raid1 D2 raid1 D5 --> MD0
D3 raid1 D4 raid1 D6 --> MD1
MD0 raid0 MD1 --> MDF (final)

thanks , hope my question is clear now.


Regds
mallah.




>
> In the stripe of mirrors you can lose up to half of the disks and still be
> operational. In the mirror of stripes, the most you could lose is two
> drives. The performance of the two should be similar - perhaps the seek
> performance would be different for high concurrent use in PG.
>
> - Luke
>
>
> On 5/29/07 2:14 PM, "Rajesh Kumar Mallah" <mallah.rajesh@gmail.com> wrote:
>
> > hi,
> >
> > this is not really postgresql specific, but any help is appreciated.
> > i have read more spindles the better it is for IO performance.
> >
> > suppose i have 8 drives , should a stripe (raid0) be created on
> > 2 mirrors (raid1) of 4 drives each OR should a stripe on 4 mirrors
> > of 2 drives each be created ?
> >
> > also does single channel or dual channel controllers makes lot
> > of difference in raid10 performance ?
> >
> > regds
> > mallah.
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 9: In versions below 8.0, the planner will ignore your desire to
> > choose an index scan if your joining column's datatypes do not
> > match
> >
>
>
>

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Hi Rajesh,

On 5/29/07 7:18 PM, "Rajesh Kumar Mallah" <mallah.rajesh@gmail.com> wrote:

> D1 raid1 D2 raid1 D5 --> MD0
> D3 raid1 D4 raid1 D6 --> MD1
> MD0 raid0 MD1 --> MDF (final)

AFAIK you can't RAID1 more than two drives, so the above doesn't make sense
to me.

- Luke



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* Luke Lonergan (llonergan@greenplum.com) wrote:
> Hi Rajesh,
>
> On 5/29/07 7:18 PM, "Rajesh Kumar Mallah" <mallah.rajesh@gmail.com> wrote:
>
> > D1 raid1 D2 raid1 D5 --> MD0
> > D3 raid1 D4 raid1 D6 --> MD1
> > MD0 raid0 MD1 --> MDF (final)
>
> AFAIK you can't RAID1 more than two drives, so the above doesn't make sense
> to me.

It's just more copies of the same data if it's really a RAID1, for the
extra, extra paranoid. Basically, in the example above, I'd read it as
"D1, D2, D5 have identical data on them".

Thanks,

Stephen

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Stephen,

On 5/29/07 8:31 PM, "Stephen Frost" <sfrost@snowman.net> wrote:

> It's just more copies of the same data if it's really a RAID1, for the
> extra, extra paranoid. Basically, in the example above, I'd read it as
> "D1, D2, D5 have identical data on them".

In that case, I'd say it's a waste of disk to add 1+2 redundancy to the
mirrors.

- Luke



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On 5/29/07, Luke Lonergan <llonergan@greenplum.com> wrote:
> AFAIK you can't RAID1 more than two drives, so the above doesn't make sense
> to me.

Yeah, I've never seen a way to RAID-1 more than 2 drives either. It
would have to be his first one:

D1 + D2 = MD0 (RAID 1)
D3 + D4 = MD1 ...
D5 + D6 = MD2 ...
MD0 + MD1 + MD2 = MDF (RAID 0)

--
Jonah H. Harris, Software Architect | phone: 732.331.1324
EnterpriseDB Corporation | fax: 732.331.1301
33 Wood Ave S, 3rd Floor | jharris@enterprisedb.com
Iselin, New Jersey 08830 | http://www.enterprisedb.com/

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On Wed, 30 May 2007, Jonah H. Harris wrote:

> On 5/29/07, Luke Lonergan <llonergan@greenplum.com> wrote:
>> AFAIK you can't RAID1 more than two drives, so the above doesn't make
>> sense
>> to me.
>
> Yeah, I've never seen a way to RAID-1 more than 2 drives either. It
> would have to be his first one:
>
> D1 + D2 = MD0 (RAID 1)
> D3 + D4 = MD1 ...
> D5 + D6 = MD2 ...
> MD0 + MD1 + MD2 = MDF (RAID 0)
>

I don't know what the failure mode ends up being, but on linux I had no
problems creating what appears to be a massively redundant (but small) array

md0 : active raid1 sdo1[10](S) sdn1[8] sdm1[7] sdl1[6] sdk1[5] sdj1[4] sdi1[3] sdh1[2] sdg1[9] sdf1[1] sde1[11](S) sdd1[0]
896 blocks [10/10] [UUUUUUUUUU]

David Lang

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On 30/05/07, david@lang.hm <david@lang.hm> wrote:
>
> On Wed, 30 May 2007, Jonah H. Harris wrote:
>
> > On 5/29/07, Luke Lonergan <llonergan@greenplum.com> wrote:
> >> AFAIK you can't RAID1 more than two drives, so the above doesn't make
> >> sense
> >> to me.
> >
> > Yeah, I've never seen a way to RAID-1 more than 2 drives either. It
> > would have to be his first one:
> >
> > D1 + D2 = MD0 (RAID 1)
> > D3 + D4 = MD1 ...
> > D5 + D6 = MD2 ...
> > MD0 + MD1 + MD2 = MDF (RAID 0)
> >
>
> I don't know what the failure mode ends up being, but on linux I had no
> problems creating what appears to be a massively redundant (but small)
> array
>
> md0 : active raid1 sdo1[10](S) sdn1[8] sdm1[7] sdl1[6] sdk1[5] sdj1[4]
> sdi1[3] sdh1[2] sdg1[9] sdf1[1] sde1[11](S) sdd1[0]
> 896 blocks [10/10] [UUUUUUUUUU]
>
> David Lang
>
>
Good point, also if you had Raid 1 with 3 drives with some bit errors at
least you can take a vote on whats right. Where as if you only have 2 and
they disagree how do you know which is right other than pick one and hope...
But whatever it will be slower to keep in sync on a heavy write system.

Peter.

* Peter Childs (peterachilds@gmail.com) wrote:
> Good point, also if you had Raid 1 with 3 drives with some bit errors at
> least you can take a vote on whats right. Where as if you only have 2 and
> they disagree how do you know which is right other than pick one and hope...
> But whatever it will be slower to keep in sync on a heavy write system.

I'm not sure, but I don't think most RAID1 systems do reads against all
drives and compare the results before returning it to the caller... I'd
be curious if I'm wrong.

Thanks,

Stephen

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