[Wylug-help] RAID, mdadm and df

[Roger]

My server uses RAID 1 for robust storage, but lately I've had some 
problems with the component drives, so after a few years while it all 
'just worked' (and I forgot what I learned) I've now had to do stuff...

The RAID was 2 x 750Gig SATA II drives (Seagate).

One of them developed a fault, so I plugged in 2 1TB drives (more 
Seagate SATA II), to give a spare as well as replacing the failed drive.

The partitioning was very simple:
md0 - sd?1    257008 blocks, type fd (raid autodetect) mounted as /boot
       sd?2   2048287 blocks, type 82 (swap)
md1 - sd?3 730266705 blocks, type fd (raid autodetect) mounted as /

On the 1TB drives, partitions 1 & 2 were identical, but partition 3 was 
974454592 blocks.

Soon, a fault appeared on the other 750 Gig, so I used mdadm to --fail 
then --remove that drive.  Without the smaller drive I then used --grow 
to increase the space on md1.

Now,
  cat /proc/mdstat
reports the full 974454592 blocks on md1, but df still reports only 
707395896 blocks.  It isn't the discrepancy between 707395896 and 
730266705 that bothers me - I know that difference is to hold the 
superblock - it's the difference between 707395896 & 974454592; the 
extra 250 Gig.

I priced what's now available and ordered a 1.5TB drive, planning that 
when it arrived, I'd replace the old sda drive, thinking in my 
ignorance, that that would allow the other drives to stay at their 
current locations, so sdb remained sdb and sdc ditto.

Between placing that order and collecting it, one of the 1TB drives 
seems to have developed a fault (in sdc1 - the /boot partition), so I've 
bought two of the 1.5TB drives.

Finally, my questions:

1.  Does it actually matter if individual drives retain their letter 
(interface connection)?  If not does the RAID software automatically 
detect any change, or do I need to do something manually?  (Should I 
connect the 1.5TB drives to the SATA0 & SATA3 connectors, or can I put 
them on 0 & 1, and move one of the 1TB drives to 3?)

2.  Why the discrepancy between /proc/mdstat and what df reports?  The 
latest drives will more than double the original capacity of md1, but I 
want that to be reliably available.  (I have rebooted since the --grow 
command seemed to succeed, with no change.)

I'd like to know what went awry in the last replacement, before doing 
the next one...

Thanks in advance,

Roger

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