Re: Unexpected slow read for HDD cluster (good write speed)

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Den mån 20 mars 2023 kl 09:45 skrev Marc <Marc@xxxxxxxxxxxxxxxxx>:
> > While
> > reading, we barely hit the mark of 100MB/s; we would expect at least
> > something similar to the write speed. These tests are being performed in
> > a
> > pool with a replication factor of 3.
> >
> >
> You don't even describe how you test? And why would you expect something like the write speed, as you described, that is a totally different configuration.

Yes, writes hit caches and can be async, whereas reads (at least large
relevant read tests) needs to get actual data off the drives, and not
from caches.
Getting 100MB/s on data not in caches from hdd drives seems reasonable
for a simplistic test (ie, one where you request a certain amount of
data, wait for it to arrive, then read a bit more and repeat for a
number of iterations).

If you instead spin up 10 guests and have all of them do read tests,
chances are you are going to see huge speedups. Not per-guest, but in
the sum of IO that your cluster can handle, which is what ceph is
aiming for. Being a good cluster storage system for cluster usage.

May the most significant bit of your life be positive.
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