Mirror mirror of my data,
Which one will let me recover later!

I have gone back and read the My mirroring is better than your Mirroring and I'm
sorry its all nonsense!   First off, there is no such thing as hardware 
mirroring.
If you are using AIX mirroring (exact copies on multiple drives)  the mirroring 
is
done by the device driver code and LVM.   If you are using RAID5 the mirroring 
is
done in the RAID adapter using both hardware and micro code. (Micro code is an 
old
IBM term for software used on an adapter card).  The closer you are to the 
hardware
the more efficient you can make the operation.  The better you understand the
hardware the greater the chance of recovering the data.  ADSM is an application 
far
removed from the base hardware. And that is the way an application should be.
Independent of the hardware.

    First lets look at the worst case.  Power drops in the middle of a write
operation.
With this all bets are off.  This is why we have a recovery log.  Before an 
update
is made, the record is read and saved in the recovery log.  The updates are 
made to
the record and also saved as the update in the recovery log.  Now we go and 
write
to the data base. Then the data is read back and checked.  If it went onto the 
disk
correctly we now clear the entry in the recovery log. To insure that the data is
safe these are the operations we put the hardware through:
        1: Writer the recovery log:  Save the update request
        2:  Read the data base:  Save the old data
        3: Write the recovery log:  Save the merge of the update with the data 
in
the data block.  AIX writes 4k blocks of data.  Most updates only change part of
the data.
        4: Write the data to the disk.
        5: Purge the disks cache.  AIX will first search the devices cache to 
see
if the data is still there. This will save time.  We want to know what is on the
disk so we force AIX to go to the disk to get the data.
        6: Read the recovery log:  Compare the log to the data read.
        7: Write the recovery log.  Clear the entry.
                From the above you can see we did 7 I/O operations to change one
byte.   This how we protect our data from errors during update.  It's been a few
years since I did this type of work so I'm sorry if I left something out.

        Mirroring protects our data while it is static, just sitting out there.

AIX Mirroring:
        To do proper AIX mirroring you should have three or more drives.  This 
is
because AIX requires 51% of the drives to be online to do a write.  For the ADSM
recovery log we use only two drives.  If ADSM can not write to the recovery log 
it
will abort without changing the database.  AIX mirroring allows you to write to 
one
drive and at the same time read from the other.  This gives you the best
performance, but requires twice the disk space.

RAID5 Mirroring:
        Raid 5 protects the data by writing part of the data on each disk and
adding ECC (Error Correcting Codes) to each part.  If one disk fails the ECC on 
the
other disks will allow a full recovery of  the missing data.  The ECC only adds
about 20% to the total disk space required. AIX mirroring adds 100%.  There are 
no
free lunches! (Unless you work in a Casino)  Because you are using all the disks
together you can not read and write at the same time.  Because the disk motors 
run
at slightly different speeds the read/write operations will start and end at
different times for each disk.  This makes for slower disk operations.  RAID 5 
is
slow, but cheap.

I hope this adds some understanding to the mirroring question.

ex-hardware engineer
ex-diagnostic programmer
ex-AIX device driver programmer
ex-IBMer (32 years)
Never could spell
Bill Dias
AIX Systems Administrator
Brown & Sharpe

Richard Sims wrote:

> >I've always thought that hardware mirroring is more efficient than software
> >mirroring.
> >The only reason that I have heard on this list to use ADSM mirroring is far
> >fetched at best so I would suggest using hardware mirroring if possible.
>
> We've EXHAUSTIVELY discussed this in the past.  And the value of ADSM 
> mirroring
> has been demonstrated: it's not far-fetched.
>
> Newcomers:  Please have a look in the bountiful information available
> at www.adsm.org for general topics, which you can bet have been addressed
> over the years.
>
>    Richard Sims, BU