> the fact that they are optimized for benchmarks and certainly not > Ceph OSD usage patterns (with or without internal journal). Are you assuming that SSHD is causing the issue? If you could elaborate on this more, it would be helpful. Cheers, Shinobu ----- Original Message ----- From: "Lionel Bouton" <lionel-subscription@xxxxxxxxxxx> To: "Mario Giammarco" <mgiammarco@xxxxxxxxx>, ceph-users@xxxxxxxxxxxxxx Sent: Tuesday, March 1, 2016 5:29:38 AM Subject: Re: Help: pool not responding Le 29/02/2016 20:43, Mario Giammarco a écrit : > [...] > I said SSHD that is a standard hdd with ssd cache. It is 7200rpms but in > benchmarks it is better than a 10000rpm disk. Lies, damn lies and benchmarks... SSHD usually have very small flash caches (16GB or less for 500GB of data or more) and AFAIK there's no distribution supporting cache hints or to be of any use here Ceph OSD cache hint support : the drive makes the decisions about when to use the cache and you can trust only one thing: the fact that they are optimized for benchmarks and certainly not Ceph OSD usage patterns (with or without internal journal). There are probably 2 kinds of optimizations that SSHD can perform : - buffering random writes with a writeback cache algorithm targeting random writes. With only 8 to 16GB of flash this would probably mean that under heavy random write usage (typical for OSD) the flash will die very fast which would kill the entire drive or lose data and so it's unlikely that this is what they use. - write the most used data (what is first loaded on system boot and what is most used) to the flash cache to speed up the OS boot sequence and access to the most used applications or data. As OSDs don't have any recognizable pattern this is useless in most cases. So SSHD for OSD are almost certainly useless. You are better off saving money by buying more ordinary HDD SATA drives or as many HDD and a few good SSDs for journal if you can afford them. In fact if the SSHD tries to cache writes and doesn't die early in the process you may get even worse performance than a pure HDD setup because most consumer-level SSD (and probably SSHD) are absolute crap for the type of access Ceph OSD do with journals (see http://www.sebastien-han.fr/blog/2014/10/10/ceph-how-to-test-if-your-ssd-is-suitable-as-a-journal-device/ for the horror stories). Best regards, Lionel _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
 |