Date: Sun, 23 Aug 2020 19:39:58 +0200 From: Gary Jennejohn <gljennjohn@gmail.com> To: guyvert@secmail.pro Cc: freebsd-hackers@freebsd.org Subject: Re: Running BSD from RAM memory using zroot Message-ID: <20200823193958.059882be@ernst.home> In-Reply-To: <6a7da06949dbd8f1c38d5faa22f3f28b.squirrel@giyzk7o6dcunb2ry.onion> References: <6a7da06949dbd8f1c38d5faa22f3f28b.squirrel@giyzk7o6dcunb2ry.onion>
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On Sun, 23 Aug 2020 05:51:39 -0700 guyvert@secmail.pro wrote: > 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? > Take a look at mount.conf(5). It seems like this may do what you want. -- Gary Jennejohn
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