Hi Carlos,
On Thu, 1 Mar 2001, Carlos Carvalho wrote:
> Corin Hartland-Swann ([EMAIL PROTECTED]) wrote on 22 February 2001 16:36:
> >On Thu, 22 Feb 2001, Carlos Carvalho wrote:
> >> What happens if I have a raid5 array and, instead of the usual
> >> situation of one disk failing, I have the opposite, all disks stop
> >> except one?
> >
> >Unless you are using a two-disk RAID-5 array (which is really a RAID-1
> >Mirror), if all disks except one fail, the array will fail.
> >
> >> This may seem strange, but may happen when you have one power supply
> >> feeding all disks but one, and another power supply feeding the last
> >> disk. If the power supply serving most disks fails, there will be one
> >> single disk running. At reboot what happens????
> >
> >The array will not start, because one disk is not enough to reconstruct
> >the contents of the array.
>
> Yes, my doubt is about how the kernel will behave. It'd be enough for
> me to remove the UNfailed disk and force its re-creation from the
> other ones. Problem is how to do this. Maybe I can just pull its plug
> and let the array start in degraded mode. The other ones will be
> coherent, so it'll start. Then shutdown, put the other one back and
> start again.
I think that this should work - but I'm no expert...
> >If you are trying to remove the power supply as
> >a point of failure, you probably want to use RAID-10 instead of RAID-5
>
> It wastes too much disk. I can't afford a 50% loss.
What array size do you need, and how many disks can you fit into the
server? I'm a strong believer in RAID-10 over RAID-5. If you have the
space for eight 80GB UDMA disks (total cost UK£1600 = US$2250), a RAID-10
will give you a 320GB array.
> In fact my goal is to have each element in the raid5 be a raid0, each
> one using a separate scsi board and cables. I didn't mention it to try
> to focus on the startup issue.
I would recommend using more UDMA disks rather than fewer SCSI disks for
reliability, performance and cost.
- UDMA disks and SCSI disks have, broadly speaking, the same reliability.
When you are planning to allow for failures, one hot spare in a UDMA
array is better than using all SCSI disks with no hot spare.
- UDMA disks can sustain 35 MB/s or so, SCSI can sustain 45 MB/s.
_ SCSI has the advantage of faster RPMs (up to 15K), and hence faster
seeks, but at prohibitive cost. I prefer using more UDMA disks to
compensate.
- Huge difference in value between UDMA and SCSI disks (at least four
times more for bog standard SCSI). The price ranges below are for the
best UDMA disks at 20GB - 80GB capacity, versus bog standard SCSI disks
at 20GB - 74GB capacity. 15K RPM ones and the like are even more.
Price/GB (UK£) Price/GB (US$)
UDMA £2.10 - £2.50 $2.94 - $3.50
SCSI £8.90 - £11.40 $12.50 - $16.00
If you're interested, have at look at the 3Ware Escalade 6800
http://www.3ware.com/ - it's pretty cheap and will do the RAID-10 in
hardware. Don't even think about using the RAID-5 on it though - the
processor is really puny and completely unsuitable for this. It appears
that RAID-5 support was bolted on as an afterthought by the PR department
just to put on the featurelist :(
> >RAID-10 arrays also give much better read/write/seek performance than the
> >RAID-5 array.
>
> Raid1 is better in writing, not read/seek. I expect a raid5-0 will
> be the same compared to a raid1-0 as a raid5 is to raid1. I must trade
> write efficiency for di$k $pace...
Yeah, got slightly carried away there :) RAID-50 and RAID-10 should have
similar read/seek performance since it mostly comes down to the number of
disks. Of course, RAID-10 will be less processor intensive, but if you
need the space, you need the space.
Regards,
Corin
/------------------------+-------------------------------------\
| Corin Hartland-Swann | Direct: +44 (0) 20 7544 4676 |
| Commerce Internet Ltd | Mobile: +44 (0) 79 5854 0027 |
| 22 Cavendish Buildings | Tel: +44 (0) 20 7491 2000 |
| Gilbert Street | Fax: +44 (0) 20 7491 2010 |
| Mayfair | Web: http://www.commerce.uk.net/ |
| London W1K 5HJ | E-Mail: [EMAIL PROTECTED] |
\------------------------+-------------------------------------/
-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to [EMAIL PROTECTED]
