Hi Andras,
please find my answers inline.
On 2/15/2020 12:27 AM, Andras Pataki wrote:
We're considering using bluestore compression for some of our data,
and I'm not entirely sure how to interpret compression results. As an
example, one of the osd perf dump results shows:
"bluestore_compressed": 28089935,
"bluestore_compressed_allocated": 115539968,
"bluestore_compressed_original": 231079936,
Am I right to interpret this as "Originally 231079936 bytes (231GB)
were compressed to 28089935 bytes (28GB) and were stored in 115539968
bytes (115GB)". If so, why is there such a huge discrepancy between
the compressed_allocated and compressed numbers (115GB vs 28GB)?
That's right except MBs not GBs.
The discrepancy is most probably caused by write block sizes and
BlueStore's allocation granularity. For spinner drives minimal
allocation size is 64K by default.
E.g. for 128K written block which takes 24K after compression allocated
disk chunk would be 64K.
Also, ceph df detail also shows some compression stats:
POOL ID STORED OBJECTS USED
%USED MAX AVAIL QUOTA OBJECTS QUOTA BYTES DIRTY
USED COMPR UNDER COMPR
cephfs_data_ec63_c 5 1.2 TiB 311.93k 1.7 TiB
0.02 5.4 PiB N/A N/A 311.93k 116 GiB 232
GiB
Would this indicate that the compressed data takes up exactly half the
space (which coincides with what the osd's are saying)? It is odd
that the allocated numbers are always half the original numbers ...
USED COMPR column shows amount of space allocated for compressed data.
I.e. this includes compressed data plus all the allocation, replication
and erasure coding overhead.
UNDER COMPR - amount of data passed through compression (summed over all
replicas) and beneficial enough to be stored in a compressed form. Here
is another 'ceph df detail' sample output where 500K object was
compressed (partially, due to unaligned object size - 448K only). So
384K at USED COMPR is caused by 3x128K allocation units for keeping
compressed data at 3 replicas. And 1.31M at UNDER COMPR column denotes
3x448K blocks of user data compressed at 3 replicas. And this totally
required (see USED) 576K = 3x128K + 3x64K (=ROUND_UP_TO(500K - 448K, 64K))
This is ceph Nautilus with all the defaults for compression (except
turning it on for the pool in question). Any insights here would be
appreciated.
Thanks,
Andras
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