Re: [PATCH 000 of 5] md: Introduction

["Adam Kropelin" <akropel1@rochester.rr.com>]

Neil Brown wrote:
> On Saturday January 21, akropel1@rochester.rr.com wrote:
>> On the first try I neglected to read the directions and increased the
>> number of devices first (which worked) and then attempted to add the
>> physical device (which didn't work; at least not the way I intended).
>
> Thanks, this is exactly the sort of feedback I was hoping for - people
> testing thing that I didn't think to...
>
>>   mdadm --create -l5 -n3 /dev/md0 /dev/sda /dev/sdb /dev/sdc
>>
>>     md0 : active raid5 sda[0] sdc[2] sdb[1]
>>           2097024 blocks level 5, 64k chunk, algorithm 2 [3/3] [UUU]
>>
>>   mdadm --grow -n4 /dev/md0
>>
>>     md0 : active raid5 sda[0] sdc[2] sdb[1]
>>           3145536 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
>
> I assume that no "resync" started at this point?  It should have done.

Actually, it did start a resync. Sorry, I should have mentioned that. I 
waited until the resync completed before I issued the 'mdadm --add' 
command.

>>     md0 : active raid5 sdd[3] sdc[2] sdb[1] sda[0]
>>           2097024 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]
>>                                 ...should this be... --> [4/3]
>> [UUU_] perhaps?
>
> Well, part of the array is "4/4 UUUU" and part is "3/3 UUU".  How do
> you represent that?  I think "4/4 UUUU" is best.

I see your point. I was expecting some indication that that my array was 
vulnerable and that the new disk was not fully utilized yet. I guess the 
resync in progress indicator is sufficient.

>> My final test was a repeat of #2, but with data actively being
>> written
>> to the array during the reshape (the previous tests were on an idle,
>> unmounted array). This one failed pretty hard, with several processes
>> ending up in the D state.
>
> Hmmm... I tried similar things but didn't get this deadlock.  Somehow
> the fact that mdadm is holding the reconfig_sem semaphore means that
> some IO cannot proceed and so mdadm cannot grab and resize all the
> stripe heads... I'll have to look more deeply into this.

For what it's worth, I'm using the Buslogic SCSI driver for the disks in 
the array.

>> I'm happy to do more tests. It's easy to conjur up virtual disks and
>> load them with irrelevant data (like kernel trees ;)
>
> Great.  I'll probably be putting out a new patch set  late this week
> or early next.  Hopefully it will fix the issues you can found and you
> can try it again..

Looking forward to it...

--Adam