RAID10 layout question

RAID10 layout question

[Chris Jones]

Hi

I've got a fileserver at home with the following space disk space:

2 * 1TB SATA disks directly connected to the motherboard
1 * 1TB partition on a SATA disk directly connected to the motherboard
2 * 1TB SATA disks connected to the motherboard via a SATA Port Multiplier

So I have 4 * 1TB physical devices available, but two of them share a SATA cable.

I'm thinking that RAID10 is probably the way to go here, but I would really like to avoid the two disks 
behind the port multiplier being used to mirror the same blocks, since the multiplier is quite new and 
I don't know yet how well it will stand up over multiple years.

I've looked through the Linux RAID HOWTO and the mdadm/md man pages and I can't see a way to 
specify in a RAID10 setup that specific disks should not be used to mirror each other.

1) Am I wrong about that? Is it possible to directly influence the layout in that kind of detail?
2) Will I take a performance hit if I manually make two RAID1 sets and then a RAID0 of them?
3) Should I forget about the layout and just use RAID6?

Cheers,

Chris Jones
   cmsj@tenshu.net
    www.tenshu.net

Re: RAID10 layout question

[Roy Sigurd Karlsbakk]

> 2) Will I take a performance hit if I manually make two RAID1 sets and
> then a RAID0 of them?

I'd rather use separate mirrors (raid1) and use lvm on top of that - better flexibility.

> 3) Should I forget about the layout and just use RAID6?

Depends what you need. RAID-6 will give you lower IOPS, but better sequencial I/O. It will also stand *any* two drives failing, while striped mirrors (raid-10 or raid-1 with raid-0/lvm on top) will only tolerate a single drive failing per mirror. The chances of two drives failing at the same time is low, but it's rather common to see a single drive failing and then seeing bad data on another drive. If not scterc (smartctl etc) is enabled, or the timeout (/sys/block/$dev/device/timeout) hasn't been increased, a single bad sector may make md kick out a drive, which would be bad if it's rebuilding a mirror. Personally, I use RAID-6 - it's safe and fast enough for my needs.

Vennlige hilsener / Best regards

roy
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