Date: Sun, 23 Aug 2020 05:51:39 -0700 From: guyvert@secmail.pro To: freebsd-hackers@freebsd.org Subject: Running BSD from RAM memory using zroot Message-ID: <6a7da06949dbd8f1c38d5faa22f3f28b.squirrel@giyzk7o6dcunb2ry.onion>
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Hello list, I am trying to get FreeBSD to operate entirely within RAM, thus no disks. Either boot via PXE/NFS/TFTP or via a SD card or eMMC that is read-only. I now have: Filesystem Size Used Avail Capacity Mounted on /dev/md0 18M 16M 956K 95% / zroot 1.7G 512M 1.2G 29% / devfs 1.0K 1.0K 0B 100% /dev The /dev/md0 is of type preloaded, and loaded from ISO CD using mfsroot variables in /boot/loader.conf. The ZFS pool is called 'zroot' and has only one filesystem, though more can be created like zroot/var/log and set quota's on that filesystem so logs cannot claim too much RAM memory. With mount we can see the FSID, but we cannot unmount the /dev/md0 because unmounting root filesystems appears to be explicitely forbidden, as 'man umount' says: "The root file system cannot be forcibly unmounted." But that is only by current design, right? We already have another root filesystem (the zroot) and no open files should be in use on the original root filesystem (/dev/md0). Interestingly, while /dev/md0 cannot be unmounted, the actual block device can be forcibly destroyed. This appears to work without problems, because the mounted /dev/md0 is not read from or written to. But of course, i would like to get rid of it altogether, and just have the zroot filesystem and no more /dev/md0. My question to you guys is, would any of you like to work on changing this behaviour? It would allow for a neat setup where, once booted, the system runs inside RAM using a memory disk (mdconfig) with a ZFS pool on top. Once the ZFS root filesystem is populated with the base and kernel, the mountpoint is changed to root: zfs set mountpoint=/ zroot That simple, now the system runs on RAM. To reduce RAM usage, compression can be enabled, and soon zstd compression will be available. Advantages: - No more unstable system if the system disk has problems, crashes, bad sectors, corruption, whatever. - Potentially faster because we read from RAM and not from disk. - Does not wear out MicroSD/eMMC/SSD flash because it is used read-only. - Better than tmpfs because of compression options, as well as many other cool features of ZFS like snapshots etc. - RAM can be encrypted if desired by using 'geli onetime /dev/md1' and using md1.eli as the zroot disk. This would help against Cold Boot Attack. Disadvantage would be slightly longer boot time, complexity and by default about 0.5GB RAM used, which can grow as the system is used. But that is worth it for me. And you can always mount something persistent on say /var and /home and leave the rest as volatile in-RAM filesystem. To avoid double RAM caching (first the md block device, then ZFS ARC cache) the primarycache and secondarycache can be changed to prevent ZFS from caching anything and read everything from block device instead. Even better would be to adapt ZFS to allow for in-memory filesystems. Then the ARC would be the actual backing and those caches would be persistent until the pool is destroyed or the system is reboot. But that requires more work, currently i'm already excited about running BSD from RAM. But my question: is there any way to force unmount of the unneeded /dev/md0 which is preloaded md (mfsroot) ? I see two approaches: 1. Adapt the umount utility to allow force unmount of root filesystem, which is currently forbidden. Should be a one-line patch? 2. Rewrite the init program so that no preloaded md is required, but init does the work of creating md disk, zroot pool and populating the base system and kernel from archive (base.txz and kernel.txz which reside on MicroSD/eMMC/ISO). Anyone thinks this is interesting? Regards, S. Guyvert
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