Re: SVM - metattach question

From: Darren Dunham (ddunham_at_redwood.taos.com)
Date: 09/30/04 

Date: Thu, 30 Sep 2004 00:27:39 GMT

Todd Sawyers <trs16@dcx.com> wrote:
> Decided to add 1 additional slice

> metattach d10 /dev/dsk/c1t8d0s4
> Grew the FS w/ growfs. Now metastat shows
> d10: Concat/Stripe
> Size: 31458321 blocks
> Stripe 0: (interlace: 32 blocks)
> Device Start Block Dbase Reloc
> c1t8d0s1 0 No Yes
> c1t9d0s1 0 No Yes
> Stripe 1:
> Device Start Block Dbase Reloc
> c1t8d0s4 0 No Yes

> My question is how could I have added that slice into stripe zero,
> instead of creating a new 1 slice stripe ?

You can't. That would require moving all sorts of data around. Your
interlace is 32 blocks, but we'll just ignore that now and assume it's 1
block. It looks a bit like this.

       t8d0s1 t9d0s1 --relayout--> t8d0s1 t9d0s1 t8d0s4
       b0 b1 b0 b1 b2
       b2 b3 b3 b4 b5
       b4 b5 b6 b7 b8
...

To add the new slice as a 3rd column of the same stripe would require
moving almost every block of data on the first two disks. Block2 to the
new disk, block 3 to the first, block 4 to t9, block 5 to the new
disk....

Such data movement is theoretically possible, but SDS/LVM doesn't
implement it. Right now, expanding a metadevice is only done by
plonking some new blocks on at the end. You can't reconfigure or
relayout the old ones.

The language the manual uses is that instead of a "stripe" this is now a
"concatenated stripe".

Of course, given your disk names (two slices of the same disk) I wouln't
want them to be in the same slice anyway. The performance would be bad. 

-- 
Darren Dunham                                           ddunham@taos.com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
         < This line left intentionally blank to confuse you. >

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