Re: Requesting replace mode for changing a disk

SandeepKsinha
2009-05-13T05:07:40Z

On Wed, May 13, 2009 at 10:24 AM, Neil Brown <neilb@suse.de> wrote:
> 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?
>

I meant we have two copies of 100 disks on each side of the mirror.
Sorry, I am not very sure how md would handle it but say, I created
two logical volumes of 100 disks and try to make a raid1 out of it.

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

So, while creation it is always guaranteed that a resync will always
happen. I believe this can be avoided by just adding some flags. The
user can specify its intention.

> 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.
>
Actually I could not understand, what did you mean by "missing" in the
above line, which creates the array.

> NeilBrown
>

--
Regards,
Sandeep.

“To learn is to change. Education is a process that changes the learner.”
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