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When formatting a large partition (10TB) that will hold only application data, does it make sense to have 'reserved for the super user' space?

122065906 blocks (5.00%) reserved for the super user

My gut says 'no', but a professional system administrator (my former hosting vendor) insisted that this was a requirement etched_in_stone. 5% of 10TB is a whopping 500GB on a file system that is application specific storage!

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up vote 5 down vote accepted

I typically do not on a filesystem that is used for data storage. For example, if only MySQL data is stored in the filesystem, and MySQL runs as the mysql user, I do not want the data to be wasted.

For the / filesystem and other filesystems that are integral to the system's operation, that reservation is well advised.

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On data volumes I tend to set the reservation to 5 inodes (tune2fs -r 5 /dev/VG00/mysql). This leaves enough to move a file or restart a service and recover from a failure without really wasting any space. For other smaller filesystems I tend to leave the reservation at 5% until it becomes a problem, tune2fs reservation changes happen immediately so can be used to recover a system that is out of disk space or inodes until you can extend the filesystem or take whatever fix action is required. Never set it to 0 blocks reserved as you should always allow some breathing room for recovery, even having 5 inodes reserved had been enough to resolve mail spool filesystems that are out of inodes for example.

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Ah, cool. tune2fs -r is handy. Setting a % is just so coarse! – Stu Thompson Jul 21 '10 at 5:22
I filled up an unimportant filesystem with no reservation yesterday and was still able to manipulate the data on the filesystem. Your point regarding the mail queue is not entirely clear. Could you clarify? – Warner Jul 22 '10 at 5:29

When the day comes that you need it you'll be glad its there - but IME you never need more than about twice the size of the largest file. Even if the space is available on other filesystems it can be handy to have plenty of overhead to map out bad blocks on a failing disk.


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