Request for Testing!
I would appreciate it if some of you would test a new option which changes the way UML writes to your filesystem files, and post your experiences here.
Normally, UML uses the standard open/close read/write library calls. The problem with this is that it duplicates data in the host’s page cache, and impacts over all disk i/o performance. The new option maps the filesystem file directly into the page cache, so it eliminates duplicate copies of pages and wasted page cache…
You’ll notice a new “Experimental Option” in the Edit Config Profile page. To turn it on, Enable ubd=mmap, save, and reboot. You should notice it in dmesg under kernel options. (dmesg | grep mmap)
Once we’ve had a bunch of successful reports, I’ll consider enabling this for everyone.
[color=darkred][b]WARNING:[/b][/color][/size] I do NOT recommend just anyone turning this on yet. There is always a chance of data loss with a maturing feature such as this. Jeff Dike fixed the last known bug in ubd=mmap last month which ate filesystems over 4GB (so that would be a good test), so use this option with caution. There haven’t been any reports of mmap related failure on the UML mailing lists, but this isn’t a widely known option.
The current version (in the 2.4.24 kernels) has worked fine for me with mmap enabled, but your mileage may vary.