I may be wrong but CEPH won’t split it into % based on disk size.
A block is wrote of 4MB to each PG that CEPH decided to use for that I/O
and replication. Yes the 8TB ones would be used more as the size / crush
algorithm will put them higher in the “chance” list but every write will be
the same size no matter the size of the disk.
So to your answer I Guess, yes the 8TB would hold more data and do more
reading. But when it comes to a write each disk will get the same amount of
data on a write. And not 25/75 or another variation.
,Ash
On Wed, 13 Mar 2019 at 11:05 PM, tim taler <robur...@gmail.com> wrote:
> Hi all,
> how are your experiences with different disk sizes in one pool
> regarding the overall performance?
> I hope someone could shed some light on the following scenario:
>
> Let's say I mix an equal amount of 2TB and 8TB disks in one pool,
> with a crush map that tries to fill all disks to the same percentage.
>
> Assuming that all disks have roughly the same speed of let's say 100MB/s,
> wouldn't that hurt the performance?
>
> As a thought experiment let's say the pool consists of only two disks,
> one 2GB, one 8GB disk - both at 100MB/s
>
> If I put a 1GB file onto it that would would write
> 250MB to the small disk and
> 750MB to the big disk.
>
> leading to an overall write time of 7,5 sec.
>
> If my pool would consist of disks with the same size,
> than on both disks 500MB would be written,
> leading to an estimated time of only 5sec.
>
> Am I right here - in principle, not in exact numbers - or am I missing some
> hidden magic ('cause even the cache operations would take different
> times for different disk sizes, right?)
>
> TIA
> and best regards
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> ceph-users@lists.ceph.com
> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
>
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