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I'm aware that EXT4 has a max_dir_size_kb mount option to set up the max size of each directory. It doesn't have a similar option to specify the max size of individual files, though. Is somebody aware of another file system that could do this ? I checked XFS, ZFS and ReiserFS, but didn't find anything, though I'm no expert on those file systems.

To be more specific about what I try to achieve : I'd like to limit the size of individual files that can be stored on a Samba share (individual files, not global quota, e.g. prevent users from storing files exceeding a given size). Samba doesn't have an option for this, and the Linux /etc/security/limits.conf doesn't work either with Samba (see https://askubuntu.com/questions/1315369/is-obey-pam-restrictions-still-supposed-to-work-in-samba-4-ubuntu-20-04, and the answer : the Samba developers explained that file size limits set up in /etc/security/limits.conf are not enforced).

So my idea was to define that limit on the file system level, but it doesn't appear there is a file system supporting this feature.

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  • I don't think that doing it a file-system level is the way to go. ext4's tuning option is not about setting limits, it is about performance/space vs usability. There may be a fuse-fs overlay. – ctrl-alt-delor Feb 11 at 15:29
  • If Samba's configuration in /etc/pam.d included a reference to pam_limits.so would that allow enforcing? – roaima Feb 11 at 17:30
  • @roama : No, basically what Samba's devs told me was that PAM's limitations aren't intended to be enforced by Samba, at least not for this purpose.Their answer : "It should work, but not for this purpose. The session managment hook helps run things like pam_mkhomedir and the account hooks try to ensure Samba stays in line with restrictions on (eg) ssh." – ChennyStar Feb 12 at 6:12
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With Lustre 2.12 and later it is possible to specify a maximum file size limit on a per-file, per-directory, or per-filesystem basis by specifying a PFL layout that has a fixed-size component end and no "to the maximum file size" component, for example:

   $ lfs setstripe -E 1G -c 1 /path/to/<file|directory>

Files created under directory would be limited to 1GiB in size (e.g. in a log directory), and if directory is the root directory it applies to all new files in the filesystem, unless otherwise specified. That said, you probably don't want to set up Lustre for your use, unless your Samba server needs a lot of capacity/performance scaling.

It also wouldn't be very hard to add such a limit to ext4. It already has to deal with different maxbytes values depending on whether the file is block mapped (old style, 4TB limit) or extent mapped (new style, 16TB limit). Setting the s_maxbytes and s_bitmap_maxbytes values via a /sys/fs/ext4/<dev>/maxbytes parameter would be pretty straight forward and likely to be accepted upstream (you could patch your server kernel in the meantime).

The only tricky part is that the sysfs value should be used as the lower limit vs. the existing limits. It should not be possible to set maxbytes higher than the existing limits. Some code would be needed in the sysfs handler to reset s_maxbytes and/or s_bitmap_maxbytes to the old limits if the maxbytes parameter is set higher. It would be easier to code if s_sysfs_maxbytes was stored as a separate parameter in the superblock, but then it would need to be checked on each use, which is a bit slower, and it is very unlikely that these values would change at runtime. It would also need to replace all s_*_maxbytes accesses in the code with a helper, like:

loff_t ext4_inode_get_maxbytes(struct inode *inode)
{
        loff_t maxbytes;

        if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
                maxbytes = inode->i_sb->s_maxbytes;
        else
                maxbytes = EXT4_SB(inode->i_sb)->s_bitmap_maxbytes;

        if (maxbytes > EXT4_SB(inode->i_sb)->s_sysfs_maxbytes)
                maxbytes = EXT4_SB(inode->i_sb)->s_sysfs_maxbytes;

        return maxbytes;
}

Even with relatively basic coding skills this could be done by a kernel newbie, with mostly search-cut-and-replace.

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  • Never used Lustre before, I'm going to take a look at it. Why do you say "you probably don't want to set up Lustre for your use" ? Do you think it's too heavyweight to set up for a basic Samba server ? – ChennyStar Feb 12 at 6:15
  • Lustre is mainly designed for very large deployments, and is able to aggregate thousands of servers and clients, so is probably too heavyweight for your needs, but if you are serving Samba for a large number of clients I could be wrong. For small scale use, the proposed ext4 patch is probably more suitable. – LustreOne Feb 14 at 3:31

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