CODEWITHSUNDEEP
linuxmemory-managementshared-memorymmapnfs
Andrew Tomazos
Andrew Tomazos

Reputation: 68588

mmap file shared via nfs?

Scenario A:

To share a read/write block of memory between two processes running on the same host, Joe mmaps the same local file from both processes.

Scenario B:

To share a read/write block of memory between two processes running on two different hosts, Joe shares a file via nfs between the hosts, and then mmaps the shared file from both processes.

Has anyone tried Scenario B? What are the extra problems that arise in Scenario B that do not apply to Scenario A?.

Upvotes: 8

Views: 9083

Answers (3)

kofemann
kofemann

Reputation: 4413

You will need byte-range-lock for correct behavior. They are available in NFS >= v4.0.

Upvotes: 5

John3136
John3136

Reputation: 29266

I'd say scenario B has all kinds of problems (assuming it works as suggested in the comments). The most obvious is the standards concurrency issues - 2 processes sharing 1 resource with no form of locking etc. That could lead to problems... Not sure whether NFS has its own peculiar quirks in this regard or not.

Assuming you can get around the concurrency issues somehow, you are now reliant on maintaining a stable (and speedy) network connection. Obviously if the network drops out, you might miss some changes. Whether this matters depends on your architecture.

My thought is it sounds like an easy way to share a block of memory on different machines, but I can't say I've heard of it being done which makes me think it isn't so good. When I think sharing data between procs, I think DBs, messaging or a dedicated server. In this case if you made one proc the master (to handle concurrency and owning the concept -i.e. whatever this guy says is the best copy of the data) it might work...

Upvotes: 0

osgx
osgx

Reputation: 94175

Mmap will not share data without some additional actions.

If you change data in mmaped part of file, changes will be stored only in memory. They will not be flushed to the filesystem (local or remote) until msync or munmap or close or even decision of OS kernel and its FS.

When using NFS, locking and storing of data will be slower than if using local FS. Timeouts of flushing and time of file operations will vary too.

On the sister site people says that NFS may have poor caching policy, so there will be much more I/O requests to the NFS server comparing I/O request count to local FS.

Upvotes: 6

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