Re: bluestore_compression_min_blob_size vs. bluestore_compression_min_blob_size_<type>

[Adam Kupczyk <akupczyk@xxxxxxxxxx>]

Hi,

First priority for compression blob is per-pool setting (taken if set)
# ceph osd pool get test compression_min_blob_size

Second, if set, bluestore_compression_min_blob_size config,

Third is selected from *_hdd *_ssd depending on the main block device in the OSD being HDD or SSD.

This can be forced by setting bluestore_debug_enforce_settings to either 'hdd' or 'ssd' if your hardware improperly reports

# cat /sys/block/sda/queue/rotational
0

Blob is an organization unit in BlueStore
https://github.com/ceph/ceph/pull/51933

You can get compression for blocks smaller than 4K only if your main device has native block size smaller.
# cat /sys/block/nvme1n1/queue/physical_block_size
512

In such a case you can set bluestore_min_alloc_size (in effect only during mkfs) to some value smaller than 4K, but multiply of 512.

This path is not often tested, and you need to own unexpected side effects.

Best regards,

Adam Kupczyk

On Wed, 16 Aug 2023 at 19:46, Tony Liu <tonyliu0592@xxxxxxxxxxx> wrote:

Hi,

With Pacific and afterwards, which of the following two settings takes precedence?
bluestore_compression_min_blob_size    (default 0)
bluestore_compression_min_blob_size_<type>    (default 8192)

I'd assume the more specific setting takes precedence?
What's this "blob"? Is it write block?
Then, there is no compression for 4K write block?

I was not able to find it out from doc, hope someone could help to clarify.


Thanks!
Tony
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