Re: CephFS thrashing through the page cache

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Got the answer to my own question; posting here if someone else
encounters the same problem. The issue is that the default stripe size in a
cephfs mount is 4 MB. If you are doing small reads (like 4k reads in the
test I posted) inside the file, you'll end up pulling at least 4MB to the
client (and then discarding most of the pulled data) even if you set
readahead to zero. So, the solution for us was to set a lower stripe size,
which aligns better with our workloads.

Thanks and Regards,
Ashu Pachauri


On Fri, Mar 10, 2023 at 9:41 PM Ashu Pachauri <ashu210890@xxxxxxxxx> wrote:

> Also, I am able to reproduce the network read amplification when I try to
> do very small reads from larger files. e.g.
>
> for i in $(seq 1 10000); do
>   dd if=test_${i} of=/dev/null bs=5k count=10
> done
>
>
> This piece of code generates a network traffic of 3.3 GB while it actually
> reads approx 500 MB of data.
>
>
> Thanks and Regards,
> Ashu Pachauri
>
> On Fri, Mar 10, 2023 at 9:22 PM Ashu Pachauri <ashu210890@xxxxxxxxx>
> wrote:
>
>> We have an internal use case where we back the storage of a proprietary
>> database by a shared file system. We noticed something very odd when
>> testing some workload with a local block device backed file system vs
>> cephfs. We noticed that the amount of network IO done by cephfs is almost
>> double compared to the IO done in case of a local file system backed by an
>> attached block device.
>>
>> We also noticed that CephFS thrashes through the page cache very quickly
>> compared to the amount of data being read and think that the two issues
>> might be related. So, I wrote a simple test.
>>
>> 1. I wrote 10k files 400KB each using dd (approx 4 GB data).
>> 2. I dropped the page cache completely.
>> 3. I then read these files serially, again using dd. The page cache usage
>> shot up to 39 GB for reading such a small amount of data.
>>
>> Following is the code used to repro this in bash:
>>
>> for i in $(seq 1 10000); do
>>   dd if=/dev/zero of=test_${i} bs=4k count=100
>> done
>>
>> sync; echo 1 > /proc/sys/vm/drop_caches
>>
>> for i in $(seq 1 10000); do
>>   dd if=test_${i} of=/dev/null bs=4k count=100
>> done
>>
>>
>> The ceph version being used is:
>> ceph version 15.2.13 (c44bc49e7a57a87d84dfff2a077a2058aa2172e2) octopus
>> (stable)
>>
>> The ceph configs being overriden:
>> WHO       MASK  LEVEL     OPTION                                 VALUE
>>      RO
>>   mon           advanced  auth_allow_insecure_global_id_reclaim  false
>>
>>   mgr           advanced  mgr/balancer/mode                      upmap
>>
>>   mgr           advanced  mgr/dashboard/server_addr
>>  127.0.0.1    *
>>   mgr           advanced  mgr/dashboard/server_port              8443
>>     *
>>   mgr           advanced  mgr/dashboard/ssl                      false
>>      *
>>   mgr           advanced  mgr/prometheus/server_addr             0.0.0.0
>>      *
>>   mgr           advanced  mgr/prometheus/server_port             9283
>>     *
>>   osd           advanced  bluestore_compression_algorithm        lz4
>>
>>   osd           advanced  bluestore_compression_mode
>> aggressive
>>   osd           advanced  bluestore_throttle_bytes
>> 536870912
>>   osd           advanced  osd_max_backfills                      3
>>
>>   osd           advanced  osd_op_num_threads_per_shard_ssd       8
>>      *
>>   osd           advanced  osd_scrub_auto_repair                  true
>>
>>   mds           advanced  client_oc                              false
>>
>>   mds           advanced  client_readahead_max_bytes             4096
>>
>>   mds           advanced  client_readahead_max_periods           1
>>
>>   mds           advanced  client_readahead_min                   0
>>
>>   mds           basic     mds_cache_memory_limit
>> 21474836480
>>   client        advanced  client_oc                              false
>>
>>   client        advanced  client_readahead_max_bytes             4096
>>
>>   client        advanced  client_readahead_max_periods           1
>>
>>   client        advanced  client_readahead_min                   0
>>
>>   client        advanced  fuse_disable_pagecache                 false
>>
>>
>> The cephfs mount options (note that readahead was disabled for this test):
>> /mnt/cephfs type ceph
>> (rw,relatime,name=cephfs,secret=<hidden>,acl,rasize=0)
>>
>> Any help or pointers are appreciated; this is a major performance issue
>> for us.
>>
>>
>> Thanks and Regards,
>> Ashu Pachauri
>>
>
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