Re: Requesting replace mode for changing a disk

Neil Brown
2009-05-13T04:54:45Z

On Wednesday May 13, sandeepksinha@gmail.com wrote:
> Hi,
>
> On Wed, May 13, 2009 at 10:01 AM, Neil Brown <neilb@suse.de> wrote:
> > On Tuesday May 12, lrhorer@satx.rr.com wrote:
> >>
> >> But doesn't creating the array with the drive wipe the contents?  If so, it
> >> doesn't seem to me this provides much redundancy.
> >
> > No.  Creating an array does not wipe the contents.
> > It might cause a resync which will copy contents from one drive to the
> > other and I don't promise which one.
> > However if you:
> >
> Now, my question is that what if I create a RAID1 with 100 disks on each side.
> Do you mean to say that there will be unnecessary resync happening
> there as well, that too for unallocated/written data.

I'm not sure what "100 disks on each side" means.
Do you mean a raid1 across 100 devices? i.e. 100 copies of each
block?

In any case, md has no concept of unallocated/written data. Every
block is potentially meaningful and needs to be copied for resync.

I have had thoughts about keeping track of which blocks have been used
so that 'TRIM' can be passed down. But it is a long way from being a
reality.

>
> If thats the case, we surely need to handle these two situations
> differently (1) which neil mentioned (2) the one I mentioned above.
>
> Remember I referring to the case of creation.
>
> >   mdadm -C /dev/md0 --level 1 -n 2 /dev/foo missing
> >   mdadm /dev/md0 --add /dev/bar
> >
> > then the contents on /dev/foo will not be changed (except for a few K
> > at the end for the metadata) and then all of foo will be copied to
> > bar.
> >
>
> Will the create happen at the first place?

I don't understand this question, sorry.

NeilBrown
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