Theodore Ts'o wrote:
Storing the tail as an extended attribute
=========================================
Stephen and I have discussed this in the past, and the idea is a simple
one; simply store the tail as an extended attribute. There are other
filesystems that have done this, most notably NTFS (post-Windows 2000).
However, this approach is a little unsatisfying to me, since it buys us
nothing if there are no other extended attributes contained by the
filesystem, and if we are using large inodes to accelerate extended
attributes, there won't be much space for any but the smallest tails
(i.e., if we are using 4k blocks, and 512 byte inodes, the largest tail
that we could store using this method is around 350 bytes or so.)
Using this technique would only require a flag in the inode indicating
it has a fragment, so the filesystem knows to look for the extended
attribute. In theory this could also be done by checking the i_size
field, and assuming that last block in the file can never normally be a
hole, but this can be quite fragile.
Disclaimer: I am not a file system expert.
That said, I still think there are some advantages to this scheme. First
of all, 350 bytes are not all that bad. Many short files are
configuration files, and many of them are shorter than 350 bytes
(resolv.conf, hostname, hosts, etc.). On my system I find more than 300
files shorter than 512 bytes in /etc, and more than 700 in my home
directory. Some applications read many configuration files, so I would
expect this scheme to lead to a considerable speed improvement.
Moreover, there is much to be said in favour of configuration systems
that store exactly one value per file, such as tcb and Elektra.
Secondly, I expect systems that do not use extended attributes to become
increasingly rare. SELinux has become mainstream. It is included in Red
Hat/Fedora, Ubuntu, Debian etch (IIRC). Desktop software such as Beagle
uses extended attributes if available, and I expect this tendency to
increase. Desktop software is also notorious for needing vast numbers of
configuration files.
Regards
Michael Burschik
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