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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