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Lustre 1.6 Operations Manual

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8.6 Using MMP<br />

The multiple mount protection (MMP) feature protects the filesystem from being<br />

mounted more than one time simultaneously. If the filesystem is mounted, MMP<br />

also protects changes by e2fsprogs to the filesystem. This feature is very important in<br />

a shared storage environment (for example, when an OST and a failover OST share a<br />

partition).<br />

The backing filesystem for <strong>Lustre</strong>, ldiskfs, supports the MMP mechanism. A block<br />

in the filesystem is updated by a kmmpd daemon at one second intervals, and a<br />

monotonically increasing sequence number is written in this block. If the filesystem<br />

is cleanly unmounted, then a special "clean" sequence is written in this block. When<br />

mounting a filesystem, ldiskfs checks if the MMP block has a clean sequence or not.<br />

Even if the MMP block holds a clean sequence, ldiskfs waits for some interval to<br />

guard against the following situations:<br />

■ Under heavy I/O, it may take longer for the MMP block to be updated<br />

■ If another node is also trying to mount the same filesystem, there may be a ’race’<br />

With MMP enabled, mounting a clean filesystem takes at least 10 seconds. If the<br />

filesystem was not cleanly unmounted, then mounting the filesystem may require<br />

additional time.<br />

Note – The MMP feature is only supported on Linux kernel versions >= 2.6.9.<br />

Note – The MMP feature is automatically enabled by mkfs.lustre for new<br />

filesystems at format time if failover is being used and the kernel and e2fsprogs<br />

support it. Otherwise, the <strong>Lustre</strong> administrator has to manually enable this feature<br />

when the filesystem is unmounted. If failover is being used, the MMP feature is<br />

automatically enabled by mkfs.lustre.<br />

- To determine if MMP is enabled: dumpe2fs -h |grep features<br />

Example: dumpe2fs -h /dev/{mdtdev} | grep 'Inode count'<br />

- To manually disable MMP: tune2fs -O ^mmp <br />

- To manually enable MMP: tune2fs -O mmp <br />

If ldiskfs detects that a filesystem is being mounted multiple times, it reports the<br />

time when the MMP block was last updated, the node name and the device name.<br />

Chapter 8 Failover 8-15

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