Re: raid 1 and df command

On Thursday 31 August 2006 17:40, ipyasaswi@xxxxxxxxx stood up and addressed
the masses in /comp.os.linux.misc/ as follows...:

Aragorn wrote:

On Thursday 31 August 2006 17:04, ipyasaswi@xxxxxxxxx stood up and
addressed the masses in /comp.os.linux.misc/ as follows...:

I got a new server with four 80 GB disks. I create hardware raid 1
(mirroring) on first two disks. Then I created raid 1 (mirroring) on
second two disks. I installed Fedora 5 default install. After
installation from the command prompt, when I typed df -k I see a total
space of about 300 GB. How is that possible?
The OS should see only 160 GB right because the first disk is mirrored
to 2 disk and third disk is mirrored to fourth disk.
While booting up the the message does show two volume groups with 73 GB

Two groups of 73 GB sounds about right, because the Linux kernel counts
in binary fashion - i.e. using powers of two - while the disk
manufacturers provide the capacity in a decimal fashion, i.e. using
powers of ten. Therefore, a disk sold as having an 80 GB capacity could
indeed turn out about 73 GB - probably a bit more - as seen from within
the operating system environment.

On account of the 300 GB, that's possibly because your kernel allows the
physical components of a hardware RAID to be seen as individual disks.

Some RAID controllers and their drivers allow an operating system to look
beyond the RAID set-up, which allows for forensics and very low level
maintenance jobs even in hardware RAID set-ups.

It all depends on the controller and the driver module. ;-)

Thanks for the reply. I understand your answer regarding 300 GB. So,
supposing Fedora 5 can see the physical components of a hardware RAID,
is the following assumption true.

The strange thing is that /df/ reports 300 GB, because /df/ is a normal
userspace utility that shouldn't read beneath the RAID layer...

Hmm... Perhaps it's looking at the block devices for the individual disks
rather than at the block devices for the RAID arrays... Are you sure that
it's a real hardware RAID, and not a software RAID sold as a hardware RAID?
Most IDE RAID controllers are not /really/ hardware RAID controllers.

Every time I write a 2 MB file to the logical volume group, which shows
about 300 GB with df -k command, will be decremented by 4 MB. In other
words because of mirroring at hardware Raid level, that specific
logical volume group will be decremented by twice the amount.

Yes, that sounds about right... ;-)

--
With kind regards,

*Aragorn*
(registered GNU/Linux user #223157)
.

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