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Old 12-05-2008, 04:36 AM   #1
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Built software raid from whole devices instead of partitions: problem?


I set up a software RAID5 with mdadm, which is working fine. I'm using external 500GB USB drives for this.

When I tried to add a spare disk just now I noticed that I didn't assemble the RAID array using partitions on the drives (sda1, sdb1, sdc1) but the whole devices instead (sda, sdb, sdc). Adding the new drive's partition (sdd1) didn't work, because there wouldn't be enough space. Adding the whole device (-a /dev/sdd) worked.

Could this be causing a problem? It's all working fine, but this would be the most convenient time to back up the data to another drive and start over. So if there is any benefit in doing so at all I would really like to know and get it over with.

Thank you!
Old 12-08-2008, 05:20 PM   #2
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There is absolutely nothing wrong with using the entire device. IMO it makes more sense than using partitions to create the virtual disk. The only problem could be the cluster size of the file system. The larger the partition -> the larger the cluster size -> the larger the smallest amount of disk space allocated for tiny files. In other words if your cluster size is 4096 bytes then that is the smallest unit of disk space that can be allocated per file. If you have a lot of files that are less than 1024 bytes then you will waste about 3072 bytes per file. This is about 75%.

On the other hand if you have a lot of files that are very large then you won't waste a lot of disk space.

All of that can be avoided because you will create partitions on the virtual disk so you can handle the cluster size issue at that point.

Last edited by stress_junkie; 12-09-2008 at 07:34 AM.


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