Agressive selective pre-allocation

["Ragnar Kjørstad" <reiserfs@ragnark.vestdata.no>]

Hi


A lot of filesystems have files that grow slowly over time, and this is
causing serious fragmentation, and thus performance-problems in some
situations.

A defragmentation-utility could potentially solve this, but maybe there
is a better way?

Some files, for instance MBOX-type mailboxes and logfiles, are known to
the user/administrator to grow over time. It is possible to preallocate
diskspace by creating large files with just zeroes in them, but this is
not very flexible and requires the application to be awere.

An alternative would be the possibility to tell the fs about the
properties of the files. Maybe an ioctl to notify the fs that for this
particular file the fs should allocate X blocks at the time? That would
ensure that the files only get limited fragmentation and performance
stays optimal.

Does the metadata-format support blocks beeing allocated but not used
yet? Is it sufficient to add them to the extent-map, mark them free,
without changing the size of the file?

An even better way would be to mark the blocks as "preallocated" without
assigning them to the file. This would allow preallocation without
actually locking the blocks - so if the disk went full the allocator
could override the preallocation and use them for other data. 

Blocks are managed in extents in reiserfs4, right? So I assume there is
an extent-list for free blocks? So maybe one could add an extra
extent-list for "preallocated blocks", and the change the allocator to
search that list if, and only if, it is appending to file on the
previous block or if there are no blocks in the free-list?


I have no time to try to implement this right now, but I just got the
idea and figgured I should write it down while fresh in memory. Any
reason why this wouldn't work? 

Personally I think fragmentation is one of the most serious
performance-problems for filesystems, and this could potentially help a
lot at least for the slow-growing files like logfiles and mboxes.



-- 
Ragnar Kjørstad