So chiming in, I think something is definitely wrong with at *least* the
frag score.
Here's what happened so far:
1. I had 8 OSDs (all 8T HDDs)
2. I added 2 more (osd.0,1) , with Quincy defaults
3. I marked 2 old ones out (the ones that seemed to be struggling the
most with IOPS)
4. I added 2 more (osd.2,3), but this time I had previously set
bluestore_min_alloc_size_hdd to 16K as an experiment
This has all happened in the space of a ~week. That means there was data
movement into the first 2 new OSDs, then before that completed I added 2
new OSDs. So I would expect some data thashing on the first 2, but
nothing extreme.
The fragmentation scores for the 4 new OSDs are, respectively:
0.746, 0.835, 0.160, 0.067
That seems ridiculous for the first two, it's only been a week. The
newest two seem in better shape, though those mostly would've seen only
data moving in, not out. The rebalance isn't done yet, but it's almost
done and all 4 OSDs have a similar fullness level at this time.
Looking at alloc stats:
ceph-0) allocation stats probe 6: cnt: 2219302 frags: 2328003 size:
1238454677504
ceph-0) probe -1: 1848577, 1970325, 1022324588544
ceph-0) probe -2: 848301, 862622, 505329963008
ceph-0) probe -6: 2187448, 2187448, 1055241568256
ceph-0) probe -14: 0, 0, 0
ceph-0) probe -22: 0, 0, 0
ceph-1) allocation stats probe 6: cnt: 1882396 frags: 1947321 size:
1054829641728
ceph-1) probe -1: 2212293, 2345923, 1215418728448
ceph-1) probe -2: 1471623, 1525498, 826984652800
ceph-1) probe -6: 2095298, 2095298, 1000065933312
ceph-1) probe -14: 0, 0, 0
ceph-1) probe -22: 0, 0, 0
ceph-2) allocation stats probe 3: cnt: 2760200 frags: 2760200 size:
1554513903616
ceph-2) probe -1: 2584046, 2584046, 1498140393472
ceph-2) probe -3: 1696921, 1696921, 869424496640
ceph-2) probe -7: 0, 0, 0
ceph-2) probe -11: 0, 0, 0
ceph-2) probe -19: 0, 0, 0
ceph-3) allocation stats probe 3: cnt: 2544818 frags: 2544818 size:
1432225021952
ceph-3) probe -1: 2688015, 2688015, 1515260739584
ceph-3) probe -3: 1086875, 1086875, 622025424896
ceph-3) probe -7: 0, 0, 0
ceph-3) probe -11: 0, 0, 0
ceph-3) probe -19: 0, 0, 0
So OSDs 2 and 3 (the latest ones to be added, note that these 4 new OSDs
are 0-3 since those IDs were free) are in good shape, but 0 and 1 are
already suffering from at least some fragmentation of objects, which is
a bit worrying when they are only ~70% full right now and only a week old.
I did delete a couple million small objects during the rebalance to try
to reduce load (I had some nasty directories), but that was cumulatively
only about 60GB of data. So while that could explain a high frag score
if there are now a million little holes in the free space map of the
OSDs (how is it calculated?), it should not actually cause new data
moving in to end up fragmented since there should be plenty of
unfragmented free space going around still.
I am now restarting OSDs 0 and 1 to see whether that makes the frag
score go down over time. I will do further analysis later with the raw
bluestore free space map, since I still have a bunch of rebalancing and
moving data around planned (I'm moving my cluster to new machines).
On 26/05/2023 00.29, Igor Fedotov wrote:
Hi Hector,
I can advise two tools for further fragmentation analysis:
1) One might want to use ceph-bluestore-tool's free-dump command to get
a list of free chunks for an OSD and try to analyze whether it's really
highly fragmented and lacks long enough extents. free-dump just returns
a list of extents in json format, I can take a look to the output if
shared...
2) You might want to look for allocation probs in OSD logs and see how
fragmentation in allocated chunks has evolved.
E.g.
allocation stats probe 33: cnt: 8148921 frags: 10958186 size: 1704348508>
probe -1: 35168547, 46401246, 1199516209152
probe -3: 27275094, 35681802, 200121712640
probe -5: 34847167, 52539758, 271272230912
probe -9: 44291522, 60025613, 523997483008
probe -17: 10646313, 10646313, 155178434560
The first probe refers to the last day while others match days (or
rather probes) -1, -3, -5, -9, -17
'cnt' column represents the amount of allocations performed in the
previous 24 hours and 'frags' one shows amount of fragments in the
resulted allocations. So significant mismatch between frags and cnt
might indicate some issues with high fragmentation indeed.
Apart from retrospective analysis you might also want how OSD behavior
changes after reboot - e.g. wouldn't rebooted OSD produce less
fragmentation... Which in turn might indicate some issues with BlueStore
allocator..
Just FYI: allocation probe printing interval is controlled by
bluestore_alloc_stats_dump_interval parameter.
Thanks,
Igor
On 24/05/2023 17:18, Hector Martin wrote:
On 24/05/2023 22.07, Mark Nelson wrote:
Yep, bluestore fragmentation is an issue. It's sort of a natural result
of using copy-on-write and never implementing any kind of
defragmentation scheme. Adam and I have been talking about doing it
now, probably piggybacking on scrub or other operations that already
area reading all of the extents for an object anyway.
I wrote a very simply prototype for clone to speed up the rbd mirror use
case here:
https://github.com/markhpc/ceph/commit/29fc1bfd4c90dd618eb9e0d4ae6474d8cfa5dfdf
Adam ended up going the extra mile and completely changed how shared
blobs works which probably eliminates the need to do defrag on clone
anymore from an rbd-mirror perspective, but I think we still need to
identify any times we are doing full object reads of fragmented objects
and consider defragmenting at that time. It might be clone, or scrub,
or other things, but the point is that if we are already doing most of
the work (seeks on HDD especially!) the extra cost of a large write to
clean it up isn't that bad, especially if we are doing it over the
course of months or years and can help keep freespace less fragmented.
Note that my particular issue seemed to specifically be free space
fragmentation. I don't use RBD mirror and I would not *expect* most of
my cephfs use cases to lead to any weird cow/fragmentation issues with
objects other than those forced by the free space becoming fragmented
(unless there is some weird pathological use case I'm hitting). Most of
my write workloads are just copying files in bulk and incrementally
writing out files.
Would simply defragging objects during scrub/etc help with free space
fragmentation itself? Those seem like two somewhat unrelated issues...
note that if free space is already fragmented, you wouldn't even have a
place to put down a defragmented object.
Are there any stats I can look at to figure out how bad object and free
space fragmentation is? It would be nice to have some clearer data
beyond my hunch/deduction after seeing the I/O patterns and the sole
fragmentation number :). Also would be interesting to get some kind of
trace of the bluestore ops the OSD is doing, so I can find out whether
it's doing something pathological that causes more fragmentation for
some reason.
Mark
On 5/24/23 07:17, Hector Martin wrote:
Hi,
I've been seeing relatively large fragmentation numbers on all my OSDs:
ceph daemon osd.13 bluestore allocator score block
{
"fragmentation_rating": 0.77251526920454427
}
These aren't that old, as I recreated them all around July last year.
They mostly hold CephFS data with erasure coding, with a mix of large
and small files. The OSDs are at around 80%-85% utilization right now.
Most of the data was written sequentially when the OSDs were created (I
rsynced everything from a remote backup). Since then more data has been
added, but not particularly quickly.
At some point I noticed pathologically slow writes, and I couldn't
figure out what was wrong. Eventually I did some block tracing and
noticed the I/Os were very small, even though CephFS-side I was just
writing one large file sequentially, and that's when I stumbled upon the
free space fragmentation problem. Indeed, deleting some large files
opened up some larger free extents and resolved the problem, but only
until those get filled up and I'm back to fragmented tiny extents. So
effectively I'm stuck at the current utilization, as trying to fill them
up any more just slows down to an absolute crawl.
I'm adding a few more OSDs and plan on doing the dance of removing one
OSD at a time and replacing it with another one to hopefully improve the
situation, but obviously this is going to take forever.
Is there any plan for offering a defrag tool of some sort for bluestore?
- Hector
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