RAID - How do I do it?

bingobob · Posted: Sat May 12, 2007 7:03 am Post subject: RAID - How do I do it?

I'd like to move to the scary world of software RAID!

I have three HDDs in my machine...

hda - OS
hdb - data
hdd - data

hdb and hdd are full of data at the moment but it's backed up and they can be wiped, they are both 120 gig drives but are by different manufactureres and therefore are not identical geometries. hda is tiny (20 gig or something), just for the OS which I occasionally stage4 to hdb so it will be backed up.

hda - For the OS (no RAID required)
hdb and hdd will be set up in a RAID1 configuration, and should I go for performance or resilience?

I have rebuilt my kernel with RAID...

see below

bingobob · Posted: Sat May 12, 2007 9:35 am Post subject:

someone said have a look at mdadm ? that right ?

NeddySeagoon · Posted: Sat May 12, 2007 9:35 am Post subject:

bingobob,

What do you want from your raid, improved reliability or improved performace.
My own setup is raid1 for boot (to keep bootloaders happy) and raid0 for other things.

Kernel raid is very flexible, since you raid partitions, not drives. It does not even force you to use partitions the same size. The space will still be used but you won't get the raid you expected in the odd corner.

The reliability/performace trade off is your call. the setup difference is the only the raid level you choose.

Install normally onto hda. When you make your kernel, be sure to choose raid support, raid0 and raid1 built in.
Make partitions on hdb and hdd for your raid sets. (you may have several raid sets, some raid0, others raid1).
Choose equal sizes on each drive. Set the partition types to 0xfd.

Use mdadm to make the raids on the partition pairs - its make your mind up time for raid levels now.
Never refer to the partitions on hdb and hdd again - they will still be in /dev but you don't use them.
Make filesystems on /dev/mdX as you would on any other device.
Mount /dev/mdX as you would /dev/hda1
Add entries to /etc/fstab for /dev/mdX and forget all about your raid set(s) until it breaks.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

bingobob · Posted: Sat May 12, 2007 9:48 am Post subject:

wow, think i need to digest this...ok, i took the next step and emerged mdadm.

I see config files in /etc and /etc/conf.d

I see what you're saying re once I'm done I will just see one "virtual" drive, and hear you re the flexibility. I want to keep it simple and just have two partitions (one on each drive, taking the whole - or close to - i realise there is a management overhead,120 gig)..

Will I really see a huge performance boost, i doubt that, so if not i should just plump for reliability. It's pretty fast as it is, no compliants, just a fileserver. It's RAID1 for me.

By install to hda (you mean a regular emerge to my OS drive), done this.

Do I need to re partition and format (and therefore wipe) hdb and hdd before beginning with mdadm?

NeddySeagoon · Posted: Sat May 12, 2007 9:51 am Post subject:

bingobob,

If you want to test, you can set up raid1 on one pair of identically sized partitions and raid0 on whatever is left over of both drives.

edit ====

I get almost a factor of 2x speed improvement.
Make your raid sets and run

bingobob · Posted: Sat May 12, 2007 9:54 am Post subject:

now that, i don't understand! hehe..i mean i understand the first part (raid 1 on identically sized partions)

Will mdadm do the partioning for me, am more used to following install guide for partitioning, what about FS type? ext3 ok?

bb goes off to put thinking hat on.

bingobob · Posted: Sat May 12, 2007 9:56 am Post subject:

have to pop out for an hour, will hope to start building on return...
I;d like to be raided today ! am sure i have all the hardware in place (and the all important backup)..

bingobob · Posted: Sat May 12, 2007 10:04 am Post subject:

what do you mean by this...

"and raid0 on whatever is left over of both drives."

NeddySeagoon · Posted: Sat May 12, 2007 10:05 am Post subject:

bingobob,

Use fdisk to create and set the partition tables - just as per the install, with the exception of setting the type to 0xfd.
mdadm will only create the raid set on the partitions you tell it to, its not a partitioning tool.

The kernel keeps the logical data organisation on the drive (raid level or whatever) well away from the filesystem code.
ext3 is a good choice but its not related to raid. Its a good idea to use a journaled filesystem when its going to be big.
Here is my raid partition scheme.

bingobob · Posted: Sat May 12, 2007 10:07 am Post subject:

for the record here's my unraided result.

bingobob · Posted: Sat May 12, 2007 10:09 am Post subject:

ok, so i will repartition, later...

here's the output of fdisk -l (as it stands)

bingobob · Posted: Sat May 12, 2007 10:10 am Post subject:

thanks, for this btw, the mists of confusion are clearing - especially the fdisk info.

NeddySeagoon · Posted: Sat May 12, 2007 10:29 am Post subject:

bingobob,

Hmm. 27Mb/sec is not very good. Getting

bingobob · Posted: Sat May 12, 2007 11:00 am Post subject:

yes..they may well have the wrong cables on them...

so many options may as well get the best performance possible while im at it...

yes the nes 400 giger you helped me with :-)

... tar for that (pun intended...I need to get out more)...but yes maybe put some of that into the mix although had intended just to chuck backup tar balls on it...

on way home now will have to check these cables.

NeddySeagoon · Posted: Sat May 12, 2007 1:16 pm Post subject:

bingobob,

Your chipset is UDMA 133 capabale,

bingobob · Posted: Sat May 12, 2007 1:19 pm Post subject:

is this a hardware thing? (i.e. cables) or hdparm settings ?

NeddySeagoon · Posted: Sat May 12, 2007 1:48 pm Post subject:

bingobob,

Its likely to be cables, or cables fitted the wrong way round. On 80 wire cables the two ends are different but the connectors are not, so they can be interchanged. Everything still operates except the 80 wire detect.
With 40 wire cables the theoretical max data rate is 33Mb/sec and your 27Mb, is just less than that, so that fits.

Compare the wire pitch between the floppy and IDE ribbons. They should a) be different. b) The IDE cable should have two wires for every one in the floppy cable. While its nice to fix, it has no effect on setting up raid.

My nForce2 box gives me a boot up warning but there is nothing in dmesg. Do you get a warning about no 80 wire ribbon detected as the kernel detects your HDD?
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Cyker · Veteran Joined: 15 Jun 2006 Posts: 1746

I wouldn't recommend Software RAID for ATA - You've sensibly put both disks on different IDE channels which will help a lot, but hda is still sharing a channel which could be problematic during heavy load.

Software RAID puts a much bigger strain on the disk subsystem, and with ATA setups that typically run off a bus, you can run into bottleneck hell.

If you have any SATA sockets, I'd recommend building the RAID on them instead...

As for the actual creation, assuming you compiled all the right bits in and emerged mdadm, the next step is to prep the disks for RAID:

I usually use cfdisk, then setup your partiton(s) - I just make one big one spanning the whole disk. Change the TYPE to FD (Linux RAID Autodetect) - This is semi-important.
Do this for all the disks to be in the array.
The partitions should all be the same size for minimum future hassles

Next, create the array using mdadm - The command will be something like:

bingobob · Posted: Sat May 12, 2007 2:49 pm Post subject:

interesting...well there is no SATA on this system...hmm this has thrown a spannner in the works. my whole plan was to buy the external 400 gigger and then raid1 the two internal disks for some resiliaance...

its a P4 with 1 gig ram. better off without raid then?

NeddySeagoon · Posted: Sat May 12, 2007 4:03 pm Post subject:

bingobob,

An IDE bus with only one drive is not really a bus.
In the case of access to hda and hdb, accesses will have to be queued but hda is the system and programs and hdb will be data, so in practice the performance loss for using IDE will be very small, as the syatem will not be trying to read/write data at the same time as system info. If you run into swapping all bets are off but swapping is a performance killer anyway.

Note that operations on the same IDE bus cannot be overlapped but operations on different IDE busses can (and will be).
In short, you won't notice.

edit ====

The bottleneck with 7,200 rpm drives (about 50MB/sec) is the head/platter data rate - not the connecting bus.
With 15,000 rpm drives, you begin to move the bottleneck to the bus.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

bingobob · Posted: Sat May 12, 2007 6:24 pm Post subject:

I have been slightly distracted by going and buying a macbook this afternoon. oops, and yes, i do intend to dual boot it to gentoo in the future! hehe, thanks for the RAID help, going to do it a little later on.

bingobob · Posted: Sun May 13, 2007 7:24 am Post subject:

have taken the plunge, repartioned both drives as type FD - i used the maximum space.

run this command

bingobob · Posted: Sun May 13, 2007 8:14 am Post subject:

strangely...seems a bit slower !

NeddySeagoon · Posted: Sun May 13, 2007 12:30 pm Post subject:

bingobob,

The format won't make any difference. I would have expected slower writes, writes have to go to both drives separately and faster reads as both drives can be read at the same time.
Run both hdparm -tT /dev/hdd1 and hdparm -tT /dev/md0 You need to be sure that nothing else is using the disks when you do these speed tests.

As you say, the cable fix can be done at any time.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

thanny · n00b Joined: 15 Aug 2005 Posts: 55

RAID1 will offer zero performance benefit. All you get with RAID1 is reliability. And if you're not mirroring the boot drive, I have to wonder what good having a reliable data volume will be if the one and only boot volume fails.

RAID0 will definitely offer a performance benefit. Since you seem to have a method for backing your data up, you'd be better off with RAID0, which almost doubles performance while doubling the space on a given mount point.

All recent 7200rpm drives read at speeds greater than 70MB/sec near the logical beginning (the outer edges of the platter). All drives are slower near the logical end (near the spindle), because the linear speed of the data under the drive head is reduced. For IDE, having the 80-wire cable is crucial (it's still only 40 pins, but having each signal travel on two wires kills a lot of noise).

No single 15Krpm drive comes close to saturating the data bus. It still takes 3 or 4 of them to bog down a U320 SCSI bus. With SAS (the only other interface you'll see a 15Krpm drive on), each drive has its own bus, and no drive comes close to saturating its bus.