Hi,
It is about time to post Tux3 patches for review. Almost. Running the
classic kernel untar test on an older 60 MB/sec hard disk turned up a
performance gap versus Ext4 by a factor of six. Hmm, that is in the
range where it could be a basic design bug, we need to do something.
Tux3 performed fine on the actual untar, but syncing those thousands of
small files to disk was slow. The cause turned out to be multiple
issues. Two were unimplemented design features. For every file, we were
writing a btree root and a leaf block with pointers to data extents.
Instead, most btrees can be "zero depth" with just a leaf block and no
index block. And many files are just a single extent so the btree is not
needed at all. When writing lots of small files, we were transferring up
to three times more blocks than necessary.
The kernel tarball showed a metadata-to-data ratio of about 1.2. This is
way too high, it should be less than a tenth of that. After factoring
out those extra blocks, a huge performance gap still remains. This must
have something to do with seeking, but our disk layout for this load is
pretty good, so what is going on?
It turns out to be important not only to write to the right place, but
at the right time. The block scheduler tries to merge physically
contiguous requests even when submitted out of order, but if the
requests are too far apart in time, an earlier request may have already
left the queue by the time an adjacent request shows up. This causes
extra, costly seeks on spinning disks. Ideally, we want our writes
contiguous in both time and space. Then the disk hardware should be able
to make seek costs effectively disappear and stream the data out at
close to media speed.
Fixes for these issues took the form of patches from Hirofumi to
eliminate redundant btree roots and submit metadata writes in a better
order for block scheduling, and a patch from me to implement a planned
"direct extent" feature to eliminate btrees completely for small files.
Iterative improvements went something like this:
* Start: 60 seconds to sync but Ext4 only needs 10 seconds
* Eliminate most btree roots => now sync in 53 seconds
* Submit file btree together with data => sync in 19 seconds
* Eliminate btree complete for small files => 15 seconds
* Flush inode table blocks after all data => 8 seconds
We ended up with:
untar:
real 0m2.706s
user 0m0.360s
sys 0m2.160s
sync:
real 0m8.651s
user 0m0.000s
sys 0m0.024s
Ext4 takes about 4 seconds to untar and 10 seconds to sync, turning in a
respectable 50 MB/sec write bandwidth on a 62 MB/sec disk. Tux3 now
syncs at 60 MB/sec, or 97% of raw media bandwidth. So we went from 500%
slower to 23% faster, woohoo. The cost for this is that we dropped out
of sight for a few weeks. Maybe it was worth it because the performance
artifact was so big that it could have been a major design deficiency
instead of what it really was: leaving some details for later.
When we checked read performance on the untarred tree, we immediately
saw mixed results. Re-tarring the kernel tree is faster than Ext4, but
directory listing is slower by a multiple. So we need to analyze and fix
ls without breaking the good tar and untar behavior. The question is, is
it worth another delay before putting Tux3 patches up for review?
I think not. In fact, by going quiet when we hit these things, we
detract from the spectator sport aspect of open source. It might not be
any faster to work in public, but it is more fun. Plus, it engraves a
record on the internet as a guide for the next effort to invent a new
and wonderful filesystem. It is hard to overstate the value to our
project of all the historical chatter about design and development
process for Ext4 and other Linux filesystems. We often find ourselves
retracing the same learning processes. By doing this work in public, we
give something back.
Improving ls performance to Ext4 standards may just be a matter of
implementing inode table readahead, or it might be that plus something
else. In any case, this will go on the longish list of important issues
that are not central.
Inline data is a related item already on that list. There is a nice plan
for it, where the same design feature handles inline files and tail
packing, similar to extended attributes. In particular, most directories
in the kernel tarball are small enough to inline, which could speed up
ls significantly. With most files and directories inlined, a Tux3
filesystem becomes a single, fatter btree, with different issues and
tradeoffs. On the whole, I expect substantial improvements in both space
utilization and performance. The final chapter in the tar performance
saga has yet to be written.
We also need to ask why we are putting so much effort into performing
well on spinning disks, which are rapidly disappearing. Two reasons.
First, spinning disks are not gone yet, they are just migrating to a
backend storage role. Second, optimizations for spinning disk are
helpful for solid state storage more often than not. In this case,
keeping related requests close together in time lets the flash
translation layer pack its erase blocks better, reducing write
multiplication during space recovery, and in turn increasing media life
and performance. So it is too soon to forget about the idiosyncrasies
and challenges of traditional rotating media, perhaps ten or twenty
years too soon.
Regards,
Daniel
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