Yeah, that's your problem, doing a single thread rsync when you have quite poor write latency will not be quick. SSD journals should give you a fair performance boost, otherwise you need to coalesce the writes at the client so that Ceph is given bigger IOs at higher queue depths.
RBD Cache can help here as well as potentially FS tuning to buffer more aggressively. If writeback RBD cache is enabled, data will be buffered by RBD until a sync is called by the client, so data loss can occur during this period if the app is not issuing fsyncs properly. Once a sync is called data is flushed to the journals and then later to the actual OSD store. From: ceph-users [mailto:ceph-users-boun...@lists.ceph.com] On Behalf Of Piotr Wachowicz Sent: 01 May 2015 10:14 To: Nick Fisk Cc: ceph-users@lists.ceph.com Subject: Re: [ceph-users] How to estimate whether putting a journal on SSD will help with performance? Thanks for your answer, Nick. Typically it's a single rsync session at a time (sometimes two, but rarely more concurrently). So it's a single ~5GB typical linux filesystem from one random VM to another random VM. Apart from using RBD Cache, is there any other way to improve the overall performance of such a use case in a Ceph cluster? In theory I guess we could always tarball it, and rsync the tarball, thus effectively using sequential IO rather than random. But that's simply not feasible for us at the moment. Any other ways? Sidequestion: does using RBDCache impact the way data is stored on the client? (e.g. a write call returning after data has been written to Journal (fast) vs written all the way to the OSD data store(slow)). I'm guessing it's always the first one, regardless of whether client uses RBDCache or not, right? My logic here is that otherwise that would imply that clients can impact the way OSDs behave, which could be dangerous in some situations. Kind Regards, Piotr On Fri, May 1, 2015 at 10:59 AM, Nick Fisk <n...@fisk.me.uk <mailto:n...@fisk.me.uk> > wrote: How many Rsync's are doing at a time? If it is only a couple, you will not be able to take advantage of the full number of OSD's, as each block of data is only located on 1 OSD (not including replicas). When you look at disk statistics you are seeing an average over time, so it will look like the OSD's are not very busy, when in fact each one is busy for a very brief period. SSD journals will help your write latency, probably going down from around 15-30ms to under 5ms From: ceph-users [mailto:ceph-users-boun...@lists.ceph.com <mailto:ceph-users-boun...@lists.ceph.com> ] On Behalf Of Piotr Wachowicz Sent: 01 May 2015 09:31 To: ceph-users@lists.ceph.com <mailto:ceph-users@lists.ceph.com> Subject: [ceph-users] How to estimate whether putting a journal on SSD will help with performance? Is there any way to confirm (beforehand) that using SSDs for journals will help? We're seeing very disappointing Ceph performance. We have 10GigE interconnect (as a shared public/internal network). We're wondering whether it makes sense to buy SSDs and put journals on them. But we're looking for a way to verify that this will actually help BEFORE we splash cash on SSDs. The problem is that the way we have things configured now, with journals on spinning HDDs (shared with OSDs as the backend storage), apart from slow read/write performance to Ceph I already mention, we're also seeing fairly low disk utilization on OSDs. This low disk utilization suggests that journals are not really used to their max, which begs for the questions whether buying SSDs for journals will help. This kind of suggests that the bottleneck is NOT the disk. But,m yeah, we cannot really confirm that. Our typical data access use case is a lot of small random read/writes. We're doing a lot of rsyncing (entire regular linux filesystems) from one VM to another. We're using Ceph for OpenStack storage (kvm). Enabling RBD cache didn't really help all that much. So, is there any way to confirm beforehand that using SSDs for journals will help in our case? Kind Regards, Piotr
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