Hello Loic!
We have developed a strategy in pacific - reducing the min_alloc_size
for HDD to 4KB by default.
Igor Fedotov did a lot of investigation and benchmarking, and came up
with some improvements to bluestore [1][2] to make this change have
little performance impact (it even increases performance in many cases).
Josh
[0] https://github.com/ceph/ceph/pull/34588
[1] https://github.com/ceph/ceph/pull/33434
[2] https://github.com/ceph/ceph/pull/33365
On 2/14/21 9:21 AM, Loïc Dachary wrote:
Bonjour,
Reading Karan's blog post about benchmarking the insertion of billions objects to Ceph via S3 / RGW[0] from last year, it reads:
we decided to lower bluestore_min_alloc_size_hdd to 18KB and re-test. As represented in chart-5, the object creation rate found to be notably reduced after lowering the bluestore_min_alloc_size_hdd parameter from 64KB (default) to 18KB. As such, for objects larger than the bluestore_min_alloc_size_hdd , the default values seems to be optimal, smaller objects further require more investigation if you intended to reduce bluestore_min_alloc_size_hdd parameter.
There also is a mail thread dated 2018 on this topic as well, with the same conclusion although using RADOS directly and not RGW[3]. I read the RGW data layout page in the documentation[1] and concluded that by default every object inserted with S3 / RGW will indeed use at least 64kb. A pull request from last year[2] seems to confirm it and also suggests modifying bluestore_min_alloc_size_hdd has adverse side effects.
That being said, I'm curious to know if people developed strategies to cope with this overhead. Someone mentioned packing objects together client side to make them larger. But maybe there are simpler ways to do the same?
Cheers
[0] https://www.redhat.com/en/blog/scaling-ceph-billion-objects-and-beyond
[1] https://docs.ceph.com/en/latest/radosgw/layout/
[2] https://github.com/ceph/ceph/pull/32809
[3] https://www.spinics.net/lists/ceph-users/msg45755.html
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