Running fsck on Large Paritions in FreeBSD

If you need to run fsck on a large FreeBSD partition, you might have a bit of trouble. fsck needs a fair bit of memory to do its work, and if you don't have enough, you'll run into difficulties getting to to complete. Here are some steps to help get fsck to run:

If fsck runs out of memory, you'll see something like

fsck_ufs: cannot alloc 30326 bytes for inoinfo

or

fsck_ufs: cannot increase directory list

You need to tell the kernel that it can use more memory for a single process than it otherwise could. This is accomplished by setting kern.maxdsiz in /boot/loader.conf. By default, this 'datasize' value is set at half the installed RAM size:

[root@bergen ~]# limits
Resource limits (current):
  cputime          infinity secs
  filesize         infinity kB
  datasize           524288 kB
  stacksize           65536 kB
  coredumpsize     infinity kB
  memoryuse        infinity kB
  memorylocked     infinity kB
  maxprocesses         1789
  openfiles            3578
  sbsize           infinity bytes
  vmemoryuse       infinity kB

In /boot/loader.conf, you'll want to add:

kern.maxdsiz="2684354560" # 2.5GB

What values you use are up to you, but the general rule of thumb is that you need 1GB for every 1TB of partition that is filled with data. In my experience on FreeBSD 6.3, the actual number seems to be a little less than that, as I was able to fsck a 3.5TB filesystem with 2.5GB allocated.

If you have extra RAM sitting around, install it. fsck can use swap space, but that makes it run really slowly, and it is best if it can keep its data in RAM. The more RAM you have, the better.

There is one other limit to worry about. If you're using the 32-bit i386 version of FreeBSD, your system cannot by default access more than 4GB of RAM, as that is the limit of the 32-bit address space. If you set kern.maxdsiz at, beyond, or even close to 4294967296, your system may crash on boot or during fsck. Supposedly, recompiling your kernel with PAE extensions can fix this, but I've yet to try it.

If you accidentally set the limit too high, you can unset it via the boot loader:

• When the FreeBSD boot menu comes up, say 'Escape to Boot Prompt'
• Type unset kern.maxdsiz
• Type boot