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List:       ceph-users
Subject:    [ceph-users] Re: Bluestore does not defer writes
From:       Dennis Benndorf <dennis.benndorf () googlemail ! com>
Date:       2020-08-31 15:11:47
Message-ID: e76ac9b3d405988c5ed3dad2c847e8297a9bde31.camel () googlemail ! com
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Here the blog I wrote about:

https://yourcmc.ru/wiki/index.php?title=Ceph_performance&mobileaction=toggle_view_desktopHDD
 for data + SSD for journal
Filestore writes everything to the journal and only starts to flush 
it to the data device when the journal fills up to the configured 
percent. This is very convenient because it makes journal act as a 
 «temporary buffer » that absorbs random write bursts.

Bluestore can't do the same even when you put its WAL+DB on SSD. 
It also has sort of a  «journal » which is called  «deferred write
queue », 
but it's very small (only 64 requests) and it lacks any kind of 
background flush threads. So you actually can increase the maximum 
number of deferred requests, but after the queue fills up the 
performance will drop until OSD restarts.




Maybe I have been hitten by that. Is there any change planned or done
on this?
Cant imagine users regulary restart their osds to get performance
back....

Regards,
Dennis


-------- Weitergeleitete Nachricht --------Von: Wido den Hollander <
wido@42on.com>An: Dennis Benndorf <dennis.benndorf@googlemail.com>, 
ceph-users@ceph.ioBetreff: Re: [ceph-users] Bluestore does not defer
writesDatum: Mon, 31 Aug 2020 16:06:37 +0200

On 31/08/2020 11:00, Dennis Benndorf wrote:
Hi,
today I recognized bad performance in our cluster. Running "watch
cephosd perf |sort -hk 2 -r" I found that all bluestore OSDs are slow
oncommit and that the commit timings are equal to their apply timings:
For exampleEvery 2.0s: ceph osd perf |sort -hk 2-
r                                                  440                 
82                82430                 58                58435        
         56                56449                 53                5344
2                 40                40441                 30           
     30439                 27                27  99                  0 
                1  98                  0                 0  97         
         0                 2  96                  0                 6  
95                  0                 2  94                  0         
        6  93                  0                13
The once with zero commit timings are filestore and the others
arebluestore osds.I did not see this after installing the new bluestore
osds (maybe thisoccured later).Both types of osds have nvmes as
journal/db. Servers have equalcpus/ram etc.
The only tuning regarding bluestore is:   bluestore_block_db_size =
69793218560   bluestore_prefer_deferred_size_hdd = 524288In order to
make a filestore like behavior, but that does not seem towork.
As far as I know is that with BlueStore apply and commit latencies are
equal.
Where did you get the idea that you could influence this with these
settings?
Wido

Any tips?
Regards Dennis_______________________________________________ceph-users 
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