The NOVA filesystem [LWN.net]

The NOVA filesystem

Posted Aug 5, 2017 22:42 UTC (Sat) by pr1268 (guest, #24648)
Parent article: The NOVA filesystem

it is impossible to move a NOVA filesystem from one system to another if the two machines do not have the same number of CPUs.

Do you suppose that an option to limit the number of per-CPU arrays of inodes when creating a NOVA filesystem can be implemented?

For example: % mknovafs -O cpu=4 /dev/nvramdevice1 (or similar) such that the number of cpus may be less than how many CPUs that computer has.

Setting cpu=1 could make a filesystem capable of being exported to any computer. Of course, that would not make sense if your whole purpose is to exploit that technology to its fullest (quoting our Editor).

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The NOVA filesystem

Posted Aug 6, 2017 7:50 UTC (Sun) by swanson (guest, #116493) [Link] (4 responses)

We don't need anything so complicated as a separate utility.

The origin of the CPU-count dependence is that NOVA divides PMEM into per-CPU allocation regions. We use the current CPU ID as a hint about which region to use and avoid contention on the locks that protect it.

So moving from a smaller number of CPUs to a larger number of CPUs just means more contention for the locks. Moving from a larger number to a smaller number is no problem at all. So, our current plan is to set the CPU count very high (like 256) when the file system is created.

-steve

The NOVA filesystem

Posted Aug 6, 2017 14:42 UTC (Sun) by jhoblitt (subscriber, #77733) [Link]

This is vaguely similar to XFS' allocation groups?

Number of CPUs

Posted Aug 7, 2017 7:49 UTC (Mon) by skitching (guest, #36856) [Link]

That sounds to me like "bucketing" as used in NoSQL databases or "consistent hashing" algorithms.

The NOVA filesystem

Posted Aug 7, 2017 8:50 UTC (Mon) by alonz (subscriber, #815) [Link] (1 responses)

Does this mean some memory cannot be allocated unless you have all 256 CPUs?

The NOVA filesystem

Posted Aug 7, 2017 9:20 UTC (Mon) by Cyberax (✭ supporter ✭, #52523) [Link]

I would imagine that each CPU will have several partitions assigned to it. If the number of CPUs is less than the number of partitions then some CPUs will have multiple partitions, if the reverse is true then some partitions will be shared across several CPUs.