RE: Mirrored Raid 5 responses


Subject: RE: Mirrored Raid 5 responses
From: Wadell, Jim S (SAIC) (WadellJS@BP.com)
Date: Wed Nov 05 2003 - 06:59:42 AKST


After looking up the system on the Dell site, you have a dedicated dual =
channel RAID controller and "up to" 14 drive storage array. The 220S is =
on the Rddhat site, thought I do not find the box itself. This means =
that Linux should have a driver which will work with
the Raid controller. As soon as the software is installed, the kernel =
does not care if the hardware behind the driver is raid, ide, scsi, or =
tin can and strings. If the raid array fails, it should not be the =
problem of the software, only flaky or bad hardware.

I would install Linux (red hat, debian, or slackware, my prejudices are =
showing), set up a raid 5 array for data, raid 1 for the o/s (assuming =
it is possible), then fire it up, load data, and set up scripts on =
several clients to beat the snot out of it. If it fails, there is a Dell =
problem.

Just my $.02

Jim

-----Original Message-----
From: volz [mailto:volz@koyukuk.at.uaa.alaska.edu]
Sent: Tuesday, November 04, 2003 5:02 PM
To: aklug@aklug.org; Wadell, Jim S (SAIC)
Subject: RE: Mirrored Raid 5 responses

Mike, Mac, Dee and Jim,

Thank you for your responses.

I did not give enough detail.

We have a DELL 755N NAS box with a perc 3DC and an attached 220S. The =
system is=20
running Windows powered OS. WE have tried both a single large volume =
with=20
dedicated hot spare and two smaller volumes with global hot spare. Both =
have=20
gone down in flames with multiple drive failures. It has been an =
unreliable=20
system and does not function well as an NFS server. Furthermore, with =
the help=20
of DELL level 1 technicians we have hosed the data twice.

So we have pretty good hardware I would just like to put together a more =

resilient system that runs an operating system I understand and can give =
us the=20
type of performance we paid for.

So to address some of your answers and help me sort out a good plan, I =
would=20
like to see if I understand the issues and capabililties of the linux =
kernel and=20
lvm.

>From: "Wadell, Jim S (SAIC)" <WadellJS@BP.com>
>To: "W.D. McKinney" <deem@wdm.com>, "volz" =
<volz@koyukuk.at.uaa.alaska.edu>
>
>Last part first. Raid 4 is similar to raid 5, except all parity is on =
one disk,=20
date on the rest. Quick google indicates that Linux software raid does =
do raid4.=20
>Downside is that the parity disk gets beat up pretty bad. There are no=20
additional security (resiliency) implications that I know about. You =
survive a=20
single disk.=20
>

Are you saying that despite the fact that parity is on its own disk only =
one of=20
the data disks can go bad at a time? This is how I understand RAID4. So =
there=20
would be no gain. Toss that one.

>Again a quick google indicated some work in Kernel.org about raid15 =
(2001),=20
doing raid5 out of raid1 pairs. I knod of get the impression that it was =
an "I=20
did it because I can" sort of deal.=20
>

I was considering more of a RAID 51 scenario where if you hose the raid =
you have=20
a way to restore in hours versus days of tape restore. I see how RAID15 =
would be=20
prefereable because you could rebuild from mirror in a short time. Seems =
overly=20
expensive of drive space. Put that one aside for the moment.=20

>Assuming that you want to survive something other than physical =
destruction of=20
the box (always hard because of power, etc), I would lean toward raid5 =
with some=20
hot spares. Assuming that you do not loose two disks simultaneously, you =
just=20
keep going. I might entertain a three disk raid1 set for the / and /usr =
/var=20
stuff. However, when you come up with scenarios in which you are going =
to loose=20
more than a single disk, due to just disk failure, then you are =
envisioning=20
power (dual or triple power supply box with separate circuit/ups units), =

controller failure (1/2 to 1/3 or ? of the drives), etc. Power would be =
most=20
likely, controllers less so.=20

Hopefully the system will be on the NAS box and the raid will be a bunch =
of=20
disks for data only. THe system disks would likely be on a separate =
controller=20
with RAID1 for the system. The disk enclosure has redundant power =
supplies with=20
stout UPS and powerchute network shutdown on the server.

We have had multiple disk failures. A case of a failed disk pulling a =
neighbor=20
down I think. I attribute it to Array Manager the disk manger supplied =
with hhe=20
system.

 My guess is the best thing we could do is do two 5 disk RAID5 volumes =
with a=20
hotspare on each. That way the chance of failing two disks at a time are =
less=20
tha two out of ten and we don't get the performance hit of ten spindles =
on a=20
volume unless data is written to both volumes simultaneously. Pretty =
much our=20
current configuration but implemented through linux instead of windows.
  =20
>
>Is this what you were looking for?
>

Yes thank you.

- Karl

>Jim
>
>-----Original Message-----
>From: W.D. McKinney [mailto:deem@wdm.com]
>Sent: Tuesday, November 04, 2003 12:56 PM
>To: volz; Wadell, Jim S (SAIC)
>Subject: Re: Mirrored Raid 5
>
>
>On Tue, 2003-11-04 at 12:45, volz wrote:
>> Hello,
>>=20
>> Does anyone have experience with mirrored raid 5's? Can linux scsi =
drivers=20
and=20
>> logical volume mangement encompass such a thing? Variously called =
15, 5+1.=20
>> Anybody know where that documentation lies? I googled "RAID 5 mirror" =
and =20
got=20
>> some hits but haven't found the substantive info. There is even a =
newsgroup=20
>> post by someone using redhat 7.3 and the same hardware I want to use =
to build=20
a=20
>> 1 tbyte RAID 5. but no specifics
>>=20
>> Also is anyone aware of way(s) to put parity on a disk or mirrored =
disks and=20
>> stripe the data w/o parity across an array? Does this provide any =
additional=20
>> security benefit. Most importantly, would it be possible to survive =
the death=20
of=20
>> multiple array disks in this case?=20
>
>My friend Jim Wadell is the person to ask.=20
>Jim can you help please ?
>
>Dee
>
>>=20
>> Or more genrally the question is: given 12 scsi disks, a linux =
kernel, couple=20
of=20
>> fast processors and decent scsi card, how much safe storage can I =
make? (safe=20
>> --> Can we suffer two disks down?)
>>=20
>> - Karl Volz=20
>>=20
>> Research Associate=20
>> Alaska Experimental Forecast Facility
>>=20
>> volz@aeff.uaa.alaska.edu
>>=20
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>
>

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