Thanks for your reply.
> Let's consider both cases:
> Journals on SSDs - for writes, the write operation returns right after
> data lands on the Journal's SSDs, but before it's written to the backing
> HDD. So, for writes, SSD journal approach should be comparable to having
> a SSD cache tier.
Not quite, see below.
Could you elaborate a bit more?
Are you saying that with a Journal on a SSD writes from clients, before they can return from the operation to the client, must end up on both the SSD (Journal) *and* HDD (actual data store behind that journal)? I was under the impression that one of the benefits of having a journal on a SSD is deferring the write to the slow HDD to a later time, until after the write call returns to the client. Is that not the case? If so, that would mean SSD cache tier should be much faster in terms of write latency than SSD journal.
In your specific case writes to the OSDs (HDDs) will be (at least) 50%
slower if your journals are on disk instead of the SSD.
Is that because of the above -- with Journal on the same disk (HDD) as the data, writes have to be written twice (assuming no btrfs/zfs cow) to the HDD (journal, and data). Whereas with a Journal on the SSD write to the Journal and disk can be done in parallel with write to the HDD? (But still both of those have to be completed before the write operation returns to the client).
(Which SSDs do you plan to use anyway?)
Intel DC S3700
Thanks,
Piotr
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