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.

WARNING: 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.

Thanks,
-Chris

I'm game.

I just rebooted my linode with it enabled. I do nightly backups, so I am not overly concerned. Here's hoping for the best..

That went badly.

From dmesg:

EXT3-fs error (device ubd(98,32)) in ext3_reserve_inode_write: IO failure
EXT3-fs error (device ubd(98,32)) in ext3_orphan_add: IO failure
attempt to access beyond end of device
62:20: rw=0, want=1086378492, limit=525312
EXT3-fs error (device ubd(98,32)): ext3_get_inode_loc: unable to read inode block - inode=2507, block=808465534
EXT3-fs error (device ubd(98,32)) in ext3_reserve_inode_write: IO failure
attempt to access beyond end of device
62:20: rw=0, want=1086378492, limit=525312
EXT3-fs error (device ubd(98,32)): ext3_get_inode_loc: unable to read inode block - inode=2507, block=808465534
EXT3-fs error (device ubd(98,32)) in ext3_reserve_inode_write: IO failure
attempt to access beyond end of device
62:20: rw=0, want=1086378492, limit=525312
EXT3-fs error (device ubd(98,32)): ext3_get_inode_loc: unable to read inode block - inode=2507, block=808465534
EXT3-fs error (device ubd(98,32)) in ext3_reserve_inode_write: IO failure

Over and over and over, and I/O failures on files. I'm turning it back off..

Well then. Let me gather the info and send that along to the list.

-Chris

My var filesystem was pretty goofed up, but I managed to fix it, and am up and running again without mmap.

Thanks for giving it a try. Glad it wasn't too hosed... I guess there is our answer about mmap. It's hungry for filesystems!

I've posted a msg to uml-user, so we'll see what comes of it. I also removed the option from the config edit page...

Thanks,
-Chris

Not a problem, I was prepared if it had gone worse. I saw your email, I'll be interested to see if they can track down the problem.. It's an interesting option that I'd like to use on my non-linode UML's as well.

Sounds like Jeff knows what to look for (from #uml)...

13:05 < caker> jdike: anything else I can provide on the ubd=mmap corruption issue?
13:33 < jdike> caker: I've seen problems, so I've got something to chase
13:33 < jdike> caker: namely pages of zeros which shouldn't be

-Chris