On 03. jan. 2018 14:51, James Poole wrote:
Hi all,
Whilst on a training course recently I was told that 'min_size' had an
affect on client write performance, in that it's the required number of
copies before ceph reports back to the client that an object has been
written therefore setting a 'min_size' of 0 would only require a write
to be accepted by the journal before confirming it's been accepted.
This is contrary to further reading elsewhere that the 'min_size' is the
minimum number of copies required of an object to allow I/O and that
'size' is the parameter that would affect write speed i.e. desired
number of replicas.
Setting 'min_size' to 0 with a 'size' of 3 you would still have an
effective 'min_size' of 2 from:
https://raw.githubusercontent.com/ceph/ceph/master/doc/release-notes.rst
"* Degraded mode (when there fewer than the desired number of replicas)
is now more configurable on a per-pool basis, with the min_size
parameter. By default, with min_size 0, this allows I/O to objects
with N - floor(N/2) replicas, where N is the total number of
expected copies. Argonaut behavior was equivalent to having min_size
= 1, so I/O would always be possible if any completely up to date
copy remained. min_size = 1 could result in lower overall
availability in certain cases, such as flapping network partition"
Which leads to the conclusion that changing 'min_size' has nothing to do
with performance but is solely related to data integrity/resilience.
Could someone confirm my assertion is correct?
Many thanks
James
you are correct that it is related to data integrity.
the writes to a osd filestore is allways acked internally when it have
hit the journal. unrelated to size/min_size.
in normal operation, all osd's must ack the write before the write is
acked to the client: iow all 3 (size 3) must ack. and min_size is not
relevant in any case.
min_size is only relevant when a pg is degraded while being remapped or
backfilled (or degraded because of no space to remap/backfill into)
because of a osd or node failure. in that case min_size specify how many
osd's must ack the write before the write is acked to the client.
since failure is most likely when disks are stressing (eg with rebuild),
reducing min_size is just asking for corruption and data loss.
kind regards
Ronny Aasen
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