What does "Used Dev Size" mean (mdadm --detail)

taoleicn · 11-05-2009, 06:57 PM

HELLO.

In lab environment, I create /dev/md0, consisted of 3 partitions.
Each partition has 128MB.

Because the type is RAID5, there are about 256MB available for data.

After mounting /dev/md0 to /var/share/, "df -h" shows the correct size (259MB Free). But the output of "mdadm --detail /dev/md0" shows the following:
...
Array Size : 272896 (266.54 MiB 279.45 MB)
Used Dev Size : 136448 (133.27 MiB 139.73 MB)
...

What does "Used Dev Size" refer to?

THANK U.

Mr-Bisquit · 11-05-2009, 07:26 PM

Just what it says, the amount used.

taoleicn · 11-05-2009, 08:15 PM

I think the amount used should be near to zero.
I have not written any data to it, how can it use 136448 bytes?

Mr-Bisquit · 11-05-2009, 10:35 PM

Look at your first post. You state that you have three partitions, that 256M is free.
What happened to the other 128M?
I am not good with RAID.
If you can, try mounting /dev/md0
with a graphical partition tool and see what the result is.
I don't see any options in the man page for reading RAID.

taoleicn · 11-06-2009, 12:01 AM

Hmm, yes, on a second thought, I think I understand the real meaning of "Used Dev Size". It refers to "disk size that are not for data".

For RAID0, "Used Dev Size" does not exist.
For RAID1, "Used Dev Size" equals to half of total disks' size.
For RAID5, "Used Dev Size" equals to size that are used for parity.

Thanks for replying, Mr-Bisquit.