Lukas Czerner wrote:
On Mon, 26 Apr 2010, Lukas Czerner wrote:
Create an ioctl which walks through all the free extents in each
allocating group and discard those extents. As an addition to improve
its performance one can specify minimum free extent length, so ioctl
will not bother with shorter extents.
This of course means, that with each invocation the ioctl must walk
through whole file system, checking and discarding free extents, which
is not very efficient. The best way to avoid this is to keep track of
deleted (freed) blocks. Then the ioctl have to trim just those free
extents which were recently freed.
In order to implement this I have created new structure
ext4_deleted_data which represents deleted extent in per-group rbtree.
When blocks are freed, extents are stored in the tree (possibly merged
with other extents). The ioctl then can walk through the tree, take out
extents from the tree, compare them with the free extents and possibly
trim them.
Note that there is support for setting minimum extent length in ioctl
call, so it will not bother with shorter extents. Also, when the
previously deleted range is taken from the tree and it is not entirely
free, the free fragments are discarded and extents shorter than minlen
are NOT returned back to the tree to avoid fragmentation of the tree
which could lead to the big memory consumption.
But you may notice, that there is one problem. bb_bitmap_deleted does
not survive umount. To bypass the problem the first ioctl call have to
walk through whole file system trimming all free extents.
Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
For now it just ignores the small extents to avoid fragmentation. As I
said before, I agree that they should not be ignored, I just need to
figure out the way to do this efficiently.
I suggest to not start with optimisations: let's first take a look
what is going on in the simple case. Benchmarks are our friends..
Edward.
Also it was not properly tested yet.
-Lukas.
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