bluestore_csum = false
bluestore_csum_type = none
Here is the snippet of 'dstat'..
----total-cpu-usage---- -dsk/total- -net/total- ---paging-->
usr sys idl wai hiq siq| read writ| recv send| in out >
41 14 36 5 0 4| 138M 841M| 212M 145M| 0 0 >
42 14 35 5 0 4| 137M 855M| 213M 147M| 0 0 >
40 14 38 5 0 3| 143M 815M| 209M 144M| 0 0 >
40 14 38 5 0 3| 137M 933M| 194M 134M| 0 0 >
42 15 34 5 0 4| 133M 918M| 220M 151M| 0 0 >
35 13 43 6 0 3| 147M 788M| 194M 134M| 0 0 >
31 11 49 6 0 3| 157M 713M| 151M 104M| 0 0 >
39 14 38 5 0 4| 139M 836M| 246M 169M| 0 0 >
40 14 38 5 0 3| 139M 845M| 204M 140M| 0 0 >
40 14 37 5 0 4| 149M 743M| 210M 144M| 0 0 >
42 14 35 5 0 4| 143M 852M| 216M 150M| 0 0 >
For example, what last entry is saying that system (with 8 osds) is receiving 216M of data over network and in response to that it is writing total of 852M of data and reading 143M of data. At this time FIO on client side is reporting ~35K 4K RW iops.
Now, after a min or so, the throughput goes down to barely 1K from FIO (and very bumpy) and here is the 'dstat' snippet at that time..
----total-cpu-usage---- -dsk/total- -net/total- ---paging-->
usr sys idl wai hiq siq| read writ| recv send| in out >
2 1 83 14 0 0| 220M 58M|4346k 3002k| 0 0 >
2 1 82 14 0 0| 223M 60M|4050k 2919k| 0 0 >
3 1 82 13 0 0| 217M 63M|6403k 4306k| 0 0 >
2 1 83 14 0 0| 226M 54M|2126k 1497k| 0 0 >
So, system is barely receiving anything (~2M) but still writing ~54M of data and reading 226M of data from disk.
After killing fio script , here is the 'dstat' output..
----total-cpu-usage---- -dsk/total- -net/total- ---paging-->
usr sys idl wai hiq siq| read writ| recv send| in out >
2 1 86 12 0 0| 186M 66M| 28k 26k| 0 0 >
2 1 86 12 0 0| 201M 78M| 20k 21k| 0 0 >
2 1 85 12 0 0| 230M 100M| 24k 24k| 0 0 >
2 1 85 12 0 0| 206M 78M| 21k 20k| 0 0 >
Not receiving anything from client but still writing 78M of data and 206M of read.
Clearly, it is an effect of rocksdb compaction that stalling IO and even if we increased compaction thread (and other tuning), compaction is not able to keep up with incoming IO.
Thanks & Regards
Somnath
-----Original Message-----
From: Allen Samuels
Sent: Friday, June 10, 2016 8:06 AM
To: Sage Weil
Cc: Somnath Roy; Mark Nelson; Manavalan Krishnan; Ceph Development
Subject: RE: RocksDB tuning
-----Original Message-----
From: Sage Weil [mailto:sweil@xxxxxxxxxx]
Sent: Friday, June 10, 2016 7:55 AM
To: Allen Samuels <Allen.Samuels@xxxxxxxxxxx>
Cc: Somnath Roy <Somnath.Roy@xxxxxxxxxxx>; Mark Nelson
<mnelson@xxxxxxxxxx>; Manavalan Krishnan
<Manavalan.Krishnan@xxxxxxxxxxx>; Ceph Development <ceph-
devel@xxxxxxxxxxxxxxx>
Subject: RE: RocksDB tuning
On Fri, 10 Jun 2016, Allen Samuels wrote:
Checksums are definitely a part of the problem, but I suspect the
smaller part of the problem. This particular use-case (random 4K
overwrites without the WAL stuff) is the worst-case from an encoding
perspective and highlights the inefficiency in the current code.
As has been discussed earlier, a specialized encode/decode
implementation for these data structures is clearly called for.
IMO, you'll be able to cut the size of this by AT LEAST a factor of
3 or
4 without a lot of effort. The price will be somewhat increase CPU
cost for the serialize/deserialize operation.
If you think of this as an application-specific data compression
problem, here is a short list of potential compression opportunities.
(1) Encoded sizes and offsets are 8-byte byte values, converting
these too
block values will drop 9 or 12 bits from each value. Also, the ranges
for these values is usually only 2^22 -- often much less. Meaning that
there's 3-5 bytes of zeros at the top of each word that can be dropped.
(2) Encoded device addresses are often less than 2^32, meaning
there's 3-4
bytes of zeros at the top of each word that can be dropped.
(3) Encoded offsets and sizes are often exactly "1" block, clever
choices of
formatting can eliminate these entirely.
IMO, an optimized encoded form of the extent table will be around
1/4 of the current encoding (for this use-case) and will likely
result in an Onode that's only 1/3 of the size that Somnath is seeing.
That will be true for the lextent and blob extent maps. I'm guessing
this is a small part of the ~5K somnath saw. If his objects are 4MB
then 4KB of it
(80%) is the csum_data vector, which is a flat vector of
u32 values that are presumably not very compressible.
I don't think that's what Somnath is seeing (obviously some data here will sharpen up our speculations). But in his use case, I believe that he has a separate blob and pextent for each 4K write (since it's been subjected to random 4K overwrites), that means somewhere in the data structures at least one address and one length for each of the 4K blocks (and likely much more in the lextent and blob maps as you alluded to above). The encoding of just this information alone is larger than the checksum data.
We could perhaps break these into a separate key or keyspace.. That'll
give rocksdb a bit more computation work to do (for a custom merge
operator, probably, to update just a piece of the value) but for a 4KB
value I'm not sure it's big enough to really help. Also we'd lose
locality, would need a second get to load csum metadata on read, etc.
:/ I don't really have any good ideas here.
sage
Allen Samuels
SanDisk |a Western Digital brand
2880 Junction Avenue, Milpitas, CA 95134
T: +1 408 801 7030| M: +1 408 780 6416 allen.samuels@xxxxxxxxxxx
-----Original Message-----
From: Sage Weil [mailto:sweil@xxxxxxxxxx]
Sent: Friday, June 10, 2016 2:35 AM
To: Somnath Roy <Somnath.Roy@xxxxxxxxxxx>
Cc: Mark Nelson <mnelson@xxxxxxxxxx>; Allen Samuels
<Allen.Samuels@xxxxxxxxxxx>; Manavalan Krishnan
<Manavalan.Krishnan@xxxxxxxxxxx>; Ceph Development <ceph-
devel@xxxxxxxxxxxxxxx>
Subject: RE: RocksDB tuning
On Fri, 10 Jun 2016, Somnath Roy wrote:
Sage/Mark,
I debugged the code and it seems there is no WAL write going on and
working as expected. But, in the process, I found that onode size it is
writing
to my environment ~7K !! See this debug print.
2016-06-09 15:49:24.710149 7f7732fe3700 20
bluestore(/var/lib/ceph/osd/ceph-0) onode
#1:7d3c6423:::rbd_data.10186b8b4567.0000000000070cd4:head# is 7518
This explains why so much data going to rocksdb I guess. Once
compaction kicks in iops I am getting is *30 times* slower.
I have 15 osds on 8TB drives and I have created 4TB rbd image
preconditioned with 1M. I was running 4K RW test.
The onode is big because of the csum metdata. Try setting 'bluestore
csum
type = none' and see if that is the entire reason or if something else is
going
on.
We may need to reconsider the way this is stored.
s
Thanks & Regards
Somnath
-----Original Message-----
From: ceph-devel-owner@xxxxxxxxxxxxxxx
[mailto:ceph-devel-owner@xxxxxxxxxxxxxxx] On Behalf Of Somnath
Roy
Sent: Thursday, June 09, 2016 8:23 AM
To: Mark Nelson; Allen Samuels; Manavalan Krishnan; Ceph
Development
Subject: RE: RocksDB tuning
Mark,
As we discussed, it seems there is ~5X write amp on the system with 4K
RW. Considering the amount of data going into rocksdb (and thus kicking
of
compaction so fast and degrading performance drastically) , it seems it is
still
writing WAL (?)..I used the following rocksdb option for faster
background
compaction as well hoping it can keep up with upcoming writes and
writes
won't be stalling. But, eventually, after a min or so, it is stalling io..
bluestore_rocksdb_options =
"compression=kNoCompression,max_write_buffer_number=16,min_write_
buffer_number_to_merge=3,recycle_log_file_num=16,compaction_style=k
CompactionStyleLevel,write_buffer_size=67108864,target_file_size_base=6
7108864,max_background_compactions=31,level0_file_num_compaction_tri
gger=8,level0_slowdown_writes_trigger=32,level0_stop_writes_trigger=64,
num_levels=4,max_bytes_for_level_base=536870912,max_bytes_for_level
_multiplier=8,compaction_threads=32,flusher_threads=8"
I will try to debug what is going on there..
Thanks & Regards
Somnath
-----Original Message-----
From: ceph-devel-owner@xxxxxxxxxxxxxxx
[mailto:ceph-devel-owner@xxxxxxxxxxxxxxx] On Behalf Of Mark Nelson
Sent: Thursday, June 09, 2016 6:46 AM
To: Allen Samuels; Manavalan Krishnan; Ceph Development
Subject: Re: RocksDB tuning
On 06/09/2016 08:37 AM, Mark Nelson wrote:
Hi Allen,
On a somewhat related note, I wanted to mention that I had
forgotten
that chhabaremesh's min_alloc_size commit for different media types
was committed into master:
https://github.com/ceph/ceph/commit/8185f2d356911274ca679614611dc335
e3
efd187
IE those tests appear to already have been using a 4K min alloc size
due to non-rotational NVMe media. I went back and verified that
explicitly changing the min_alloc size (in fact all of them to be
sure) to 4k does not change the behavior from graphs I showed
yesterday. The rocksdb compaction stalls due to excessive reads
appear (at least on the
surface) to be due to metadata traffic during heavy small random
writes.
Sorry, this was worded poorly. Traffic due to compaction of metadata
(ie
not leaked WAL data) during small random writes.
Mark
Mark
On 06/08/2016 06:52 PM, Allen Samuels wrote:
Let's make a patch that creates actual Ceph parameters for these
things so that we don't have to edit the source code in the future.
Allen Samuels
SanDisk |a Western Digital brand
2880 Junction Avenue, San Jose, CA 95134
T: +1 408 801 7030| M: +1 408 780 6416 allen.samuels@xxxxxxxxxxx
-----Original Message-----
From: ceph-devel-owner@xxxxxxxxxxxxxxx [mailto:ceph-devel-
owner@xxxxxxxxxxxxxxx] On Behalf Of Manavalan Krishnan
Sent: Wednesday, June 08, 2016 3:10 PM
To: Mark Nelson <mnelson@xxxxxxxxxx>; Ceph Development
<ceph-
devel@xxxxxxxxxxxxxxx>
Subject: RocksDB tuning
Hi Mark
Here are the tunings that we used to avoid the IOPs choppiness
caused by rocksdb compaction.
We need to add the following options in src/kv/RocksDBStore.cc
before rocksdb::DB::Open in RocksDBStore::do_open
opt.IncreaseParallelism(16);
opt.OptimizeLevelStyleCompaction(512 * 1024 * 1024);
Thanks
Mana
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