Poor Performance WhenNumber of Files > 1M
ext3 at kalucki.com
Wed Jun 11 22:04:17 UTC 2008
Eric Sandeen wrote:
> John Kalucki wrote:
>> Performance seems to always map directly to the number of files in the
>> ext3 filesystem.
>> After some initial run-fast time, perhaps once dirty pages begin to be
>> written aggressively, for every 5,000 files added, my files created per
>> second tends to drop by about one. So, depending on the variables, say
>> with 6 RAID10 spindles, I might start at ~700 files/sec, quickly drop,
>> then more slowly drop to ~300 files/sec at perhaps 1 million files, then
>> see 299 files/sec for the next 5,000 creations, 298 files/sec, etc. etc.
>> As you'd expect, there isn't much CPU utilization, other than iowait,
>> and some kjournald activity.
>> Is this a known limitation of ext3? Is expecting to write to
>> O(10^6)-O(10^7) files in something approaching constant time expecting
>> too much from a filesystem? What, exactly, am I stressing to cause this
>> unbounded performance degradation?
> I think this is a linear search through the block groups for the new
> inode allocation, which always starts at the parent directory's block
> group; and starts over from there each time. See find_group_other().
> So if the parent's group is full and so are the next 1000 block groups,
> it will search 1000 groups and find space in the 1001st. On the next
> inode allocation it will re-search(!) those 1000 groups, and again find
> space in the 1001st. And so on. Until the 1001st is full, and then
> it'll search 1001 groups and find space in the 1002nd... etc (If I'm
> remembering/reading correctly, but this does jive with what you see.).
> I've toyed with keeping track (in the parent's inode) where the last
> successful child allocation happened, and start the search there. I'm a
> bit leery of how this might age, though... plus I'm not sure if it
> should be on-disk or just in memory.... But this behavior clearly needs
> some help. I should probably just get it sent out for comment.
This is the best explanation I've read so far. There does indeed appear
to be some O(n) behavior that is exacerbated by having many directories
in the working set (not open, just referenced often) and perhaps
moderate fragmentation. I read up on ext3 inode allocation, and the
attempt to place files in the same cylinder group as directories. Trying
to work with this system, I started on a fresh filesystem and flattened
the directory depth to just 4 levels, I've managed to boost performance
greatly, and flatten the degradation curve quite a bit.
I can get to about 2,800,000 files before performance starts to slowly
drop from a nearly constant ~1,700 file/sec. At ~4,000,000 files, I see
about ~1,500 files/sec, and afterwards I start to see the old behavior
of greater decline. By 5,500,000 files, it's down to 1,230 files/sec.
I've used 9% of the space and 8% of the inodes at this point.
Changing journal size and /proc/sys/fs/file-max had no effect. Even
dir_index had only marginal impact, as my directories have only about
300 files each.
I think the biggest factor to making performance nearly linear is the
number of directories in the working set. If this grows too large, the
linear allocation behavior is magnified, and performance drops. My
version of RHEL doesn't seem to allow tweaking of directory cache
behavior, perhaps a deprecated feature from the 2.4 days.
If I discover anything else, I'll be sure to update this thread.
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