Red Hat Bugzilla – Bug 968876
e4defrag incorrectly calculates optimum extents
Last modified: 2014-02-05 16:35:32 EST
The "Total/best extents" calculation for done in the e4defrag program doesn't seems to not consider the limit of 32768 file system blocks (see /usr/src/linux/fs/ext4/ext4_extents.h comment EXT_INIT_MAX_LEN) in an extent when computing the "best" value. It seems to presume 2G is the best extent size, but on a file system with a 4kb block size the largest available extent is 128M not 2G.
(The best extent size calculation in e4defrag seems to be based on block group size but the underlying implementaiton in the kernel is limited to a __le16 blocks and uses up one whole bit of that to flag whether the extent is initialized, which actually produces a one-block purturbation in the actual limit based on how the extent was allocated (e.g. whether falloc used FS_KEEP_SIZE or not), go go gadget purturbed math. 8-)).
-rw-r--r-- 1 rwhite rwhite 5.0G Jun 13 16:28 CentOS.qcow2
yeilds output: (where "/3" is wrong).
Total/best extents 152/3
Proposed patch available here:
It'd be great if you could take this discussion & patch to the upstream list, email@example.com. It sounds like you have a good handle on things and could advocate for your patch - and TBH, this is low enough priority I may never get to it.
How ext counts extents has always been dodgey, helpfully omitting things like indirect blocks on ext3, etc.
Consistency sounds good to me though. Either we should report "an extent" as "contiguous blocks on disk" regardless of implementation, or somehow munge the idea based on the intricacies of ext4's on-disk format.
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