Hello.
On 07/13/2016 03:31 AM, Christian Balzer wrote:
Hello,
did you actually read my full reply last week, the in-line parts,
not just the top bit?
http://www.spinics.net/lists/ceph-users/msg29266.html
On Tue, 12 Jul 2016 16:16:09 +0300 George Shuklin wrote:
Yes, linear io speed was concern during benchmark. I can not predict how
much linear IO would be generated by clients (compare to IOPS) so we
going to balance HDD-OSD per SSD according to real usage. If users would
generate too much random IO, we will raise HDD/SSD ratio, if they would
generate more linear-write load, we will reduce that number. I plan to
do it by reserving space for 'more HDD' or 'more SSD' in planned servers
- they will go to production with ~50% slot utilization.
Journal writes are always "linear", in a fashion.
And Ceph journals only sees writes, never reads.
So what your SSD sees is n sequential (with varying lengths, mind ya)
write streams and that's all.
Where n is the number of journals.
Yes, I knew this. I mean that under real load in production there going
to be too much random IO (directed toward OSD), that HDD inside of each
OSD would not be able to accept too much of linear writing (it needs to
serve random write & read in parallel, and this significantly reduce
speed of linear IO on the same device). I expect HDD to be busy enough
with random write/read without saturating SSD linear write performance.
My main concern is that random IO for OSD includes not only writes, but
reads too, and cold random read will slower HDD performance
significantly. On my previous experience, any weekly cronjob on server
with backup (or just 'find /') cause bad spikes of cold read, and that
drastically diminish HDD performance.
As I wrote last week, reads have nothing to do with journals.
Read have nothing to do with journals except one thing: if underlying
HDD is very busy with read, it will accept write operations slowly. And
because it will accept them slowly, SSD with journal would not get too
much IO of any kind.
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