[SATLUG] RAID5 Recovery - Post Mortem Review - Food For Thought

Don Wright satlug at sbcglobal.net
Fri Sep 11 19:25:55 CDT 2009


On Fri, 11 Sep 2009 15:24:31 -0700 (PDT), David Labens
<adlabens at swbell.net> wrote:

>I've now copied all the data off the RAID5 array - using the copy as a
>backup till I get backups up.  The 1 tb drive has approximately 244 gb
>of data on it (used space), and I've copied ALL the data from the array.
>
>The array consists of 4 HDDs with 250 GB each, a total array size of approx 1 TB, but approx 750 GB of usable storage space.
>
>My
>"off the top" conclusion is that the array should have approximately
>500 GB of free space on it.  But, I'm finding that the numbers don't
>add up.  Note the "Used Dev Size :
>488391680 (465.77 GiB 500.11 GB)"  And I wonder what that means?


I second the recommendation, use df -h:
>turbo:~# df -h /dev/md0

Mine shows:
>Filesystem            Size  Used Avail Use% Mounted on
>/dev/md0              459G  341G   95G  79% /data

while mdadm -D /dev/md0 shows:
>    Array Size : 488391808 (465.77 GiB 500.11 GB)
> Used Dev Size : 244195904 (232.88 GiB 250.06 GB)

Note that the used is exactly half the array size. I saw a note online
referring to that as a "bug." (The type 1 superblock values are
displayed incorrectly.) I'd just ignore that line and use df -h to see
how you're doing on space. What it _should_ be telling you is how much
of the virtual drive is allocated for partitions. In most cases that's
100%, so it doesn't really say anything. Having several partitions on a
multi-disk (md) virtual device is a recent development and few people
use it.

The difference between the Array Size (465.77 GiB) and the Filesystem
Size (459G) is the overhead of RAID and ext3, which reduces the space
available to data.  --Don

-- 
I just realized the USB stick I casually loaned somebody has more capacity
than all the disk drives at my university (and cost less than one textbook.)


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