Switch to software raid 10 ?

Lone_Wolf · 2012-03-15 12:17:32

My main system has 2x 300 GB WD raptor drives (10k drives, very fast) and 2x 1 TB samsung drives (7.5k drives, standard performance).

On the first raptor, sda1 i have /boot, / and swap .
the remaining space on sda1 along with sdb, sdc and sdd are all combined in 1 big LVM volume for /home.

Currently i am using approx 400 GB of /home , the rest is unused.
This basically means that of my available 2.6 TB, i use about 20% .
The performance is not great, and i'm thinking of switching to a raid 10 + raid 1 setup.

the partition setup will be something like this :

sda : entire drive as 1 raid partition
sdb : same as sda
sdc :2 raid partitions, sdc1 300 GB , sdc2 rest
sdd : same as sdc

sdc2 and sdd2 will become a raid 1 volume, approx 700 GB.

For the raid 10 volume i'm not sure what setup will give the best performance.

Raid 1+0

setup A
Mirror1 sda + sbd
Mirror2 sdc1+sdd1
Stripe : mirror1 + mirror2

setup B
Mirror1 : sda + sdc1
Mirror2 : sdb + sdd1

In both setups mirror1 + mirror2 are then put in a stripe.

In other words , for best performance should i put the 2 fastest drives together or separate them ?

Edit:
mixed the order of mirror and stripe, setups corrected

Last edited by Lone_Wolf (2012-03-15 18:59:15)

pseudonomous · 2012-03-15 15:44:20

You should have the two fastest drives together as a mirrored pair, since a mirrored pair cannot perform faster than the slowest drive in the mirrored pair.

Lone_Wolf · 2012-03-15 19:22:31

I made an error in the setups, edited them for clarity now.

pseudonomeus, your answer does seem logical for a mirror, but this is a stripe of mirrors.

Lets delve a bit deeper into this.

In SetupA mirror1 is fast, mirror2 is slow on both read & write .
SetupB mixes slow and fast drives in the mirrors, this should imo result in fast reads & slow writes.

let's assume we have 4 blocks of data, all same size and exactly the amount that fits on 1 physical drive, so the blocks are spread over all drives.
Block A + C are on mirror1 , block B+D are on mirror 2.

Reading all 4 blocks :
setup A retrieves block A + C fast, but lags with block B+D .

Setup B : all blocks are found on the fastest drives, so retrieval will be fast.

for reading setupB is faster.

writing

SetupA Block A+C are written fast, block B+D slow.

SetupB
all blocks need to be written on both fast and slow, so are equally slow.

for writing both setups are equal.

IF my reasoning is correct, this means that setupB with the drives mixed in the mirror gives the best performance.

Evanlec · 2012-03-16 23:10:14

I would keep the raptors in their own raid1 array, and the other drives in a raid0 or raid1 array. I would
not try to combine drives with such dramatic differences into a single raid10 array, as one will
inevitably bottleneck the other.

If you had 4-6 1TB 7200rpm sata drives (same size, speed, manufacturer), then raid10 would be a good choice.

Lone_Wolf · 2012-03-17 00:18:04

Originally this system was intended to have 8 drives in total, but the cost of that was way to high, so i settled for 2 of each type.

It looks like i have 2 alternatives :

SetupB as described above will have 600 GB with fast read, slow write and 700 GB slower in reading, equal in writing.

If i go with a simple setup, both raptors in a mirror and the 2 samsungs in another mirror :
300 GB fast read & write
1000 GB slow read and write

Both setups should give better performance then my current setup.

1 way i could maximize the performance for the simple setup is to put home on the raptor mirror,
and add the samsung mirror with a symlink to home/user/data (or similar).
The files i need good performance for would then be put directly under home/user/ .

looking at the current amount of data in my home partition, this is definitely doable.

Evanlec · 2012-03-17 09:58:48

Put the raptors together in a mirror and put your OS, and other files / apps that will benefit most from the higher speed.

Put long-term storage items (like your family photos) on the slow but large array

Lone_Wolf · 2012-03-17 14:03:29

That does seem to be the best approach, now to think about how to achieve this without loosing data/reinstall.

Guess i'll start by removing the 2 samsung disks from my LVM and build that mirror.

Looks like i'll have to find out the block size my disks use.

Last edited by Lone_Wolf (2012-03-17 14:08:14)

Arch Linux

#1 2012-03-15 12:17:32

Switch to software raid 10 ?

#2 2012-03-15 15:44:20

Re: Switch to software raid 10 ?

#3 2012-03-15 19:22:31

Re: Switch to software raid 10 ?

#4 2012-03-16 23:10:14

Re: Switch to software raid 10 ?

#5 2012-03-17 00:18:04

Re: Switch to software raid 10 ?

#6 2012-03-17 09:58:48

Re: Switch to software raid 10 ?

#7 2012-03-17 14:03:29

Re: Switch to software raid 10 ?

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