disk striping

[Fred Martin_1]

Can you stripe a disk with data on it?

I prefer to use SAM to set up volume groups and file systems, do I have to stripe from the command line?

[Sandip Ghosh · Accepted solution]

You have to create the stripe during the creation of logical volume. If you are creating the logical vol then you can't have any existing data on that area.

From command line it is easy to configure, give the -I and -i option. Have a look at the man lvcreate.

Sandip

[James R. Ferguson]

Hi Fred:

SAM offers the option of striping. Choose the "Modify LV Defaults" tab when you create the logical volume. Note that this is true striping, not extent-based stripes. Have a look at the 'lvcreate' man pages.

Regards!

...JRF...

[James R. Ferguson]

Hi (again):

With regard to your question "Can you stripe a disk with data on it?" --

No, not if you want to preserve the data. Disk striping is established when the logical volume is *created*. By definition, this means obliterating anything already there, either by a 'lvremove' of a logical volume (and hence its filesystem) or by starting from scratch with a 'pvcreate' of a physical volume and proceeding to 'vgcreate', 'lvcreate' and 'newfs'.

Regards!

...JRF...

[Fred Martin_1]

Many thanks.
Fred

[Patrick Wallek]

A bit of clarification here....

"Can you stripe a disk with data on it?"

What exactly do you mean by that?

You can have a VG (disk1, disk2 and disk3) that contains both striped and non-striped LVs. If you have LV1 that is on disk1 and what to create LV2 striped across disk1, disk2 and disk3, you can do it, but you MUST make sure that disk1 has the appropriate number of available extents so that the data is spread evenly over all 3 disks.

Each disk must have:

(LV Size / PE Size) / number of disks in stripe

as available extents.

Take an example with a VG with the three disks.

The PE Size is 4 MB
There are 2500 PEs per disk (10GB per disk)

You have LV1 created on disk1 with a size of 5000MB (used 1250 PEs).

You want to create a new LV striped across all 3 disks that is 15600 GB or 3900 PEs total (15600 MB / 4MB PE Size = 3900).

Each disks must have 1300 PEs available (3900 total PEs / 3 disks = 1300 PEs per disk) to stripe across all 3 disks.

This will FAIL since disk1 only has 1250 PE's available.

If you wanted to create a 9000MB LV striped across all 3 disks, this would work.

9000 MB / 4MB PE Size = 2250 total PEs

2250 PEs / 3 disks = 750 PEs per disk.

This WILL work because all 3 disks have at least 750 PEs available for use.

Does this make sense?

[Fred Martin_1]

Yeah sorry, I meant data in the area to be striped; i.e. three physical disks currently in one logical volume, with data.

As I understand it, I'd have to backup up the data and restore it.

[Patrick Wallek]

I see.

In that case, yes you would have to back up the data, recreate the LV with the appropriate striping options, and restore the data again.

[harry d brown jr]

Fred, if the filesystem you want to "STRIPE" is either not part of the three disks you want to "STRIPE" it across, OR if there is enough room to create an LV across the 3 disks (STRIPED), then you can COPY it from one file system to another. But to be safe, BACK it up to tape TWICE.

Second, make sure you have a good make_tape_recovery!

live free or die
harry