> -----Original Message----- > From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of > Wang, Zhiqiang > Sent: 01 September 2015 02:48 > To: Nick Fisk <nick@xxxxxxxxxx>; 'Samuel Just' <sjust@xxxxxxxxxx> > Cc: ceph-users@xxxxxxxxxxxxxx > Subject: Re: any recommendation of using EnhanceIO? > > > -----Original Message----- > > From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf > > Of Nick Fisk > > Sent: Wednesday, August 19, 2015 5:25 AM > > To: 'Samuel Just' > > Cc: ceph-users@xxxxxxxxxxxxxx > > Subject: Re: any recommendation of using EnhanceIO? > > > > Hi Sam, > > > > > -----Original Message----- > > > From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On > > > Behalf Of Samuel Just > > > Sent: 18 August 2015 21:38 > > > To: Nick Fisk <nick@xxxxxxxxxx> > > > Cc: ceph-users@xxxxxxxxxxxxxx > > > Subject: Re: any recommendation of using EnhanceIO? > > > > > > 1. We've kicked this around a bit. What kind of failure semantics > > > would > > you > > > be comfortable with here (that is, what would be reasonable behavior > > > if > > the > > > client side cache fails)? > > > > I would either expect to provide the cache with a redundant block > > device (ie > > RAID1 SSD's) or the cache to allow itself to be configured to mirror > > across two SSD's. Of course single SSD's can be used if the user accepts the > risk. > > If the cache did the mirroring then you could do fancy stuff like > > mirror the writes, but leave the read cache blocks as single copies to > > increase the cache capacity. > > > > In either case although an outage is undesirable, its only data loss > > which would be unacceptable, which would hopefully be avoided by the > > mirroring. As part of this, it would need to be a way to make sure a > > "dirty" RBD can't be accessed unless the corresponding cache is also > attached. > > > > I guess as it caching the RBD and not the pool or entire cluster, the > > cache only needs to match the failure requirements of the application its > caching. > > If I need to cache a RBD that is on a single server, there is no > > requirement to make the cache redundant across > racks/PDU's/servers...etc. > > > > I hope I've answered your question? > > > > > > > 2. We've got a branch which should merge soon (tomorrow probably) > > > which actually does allow writes to be proxied, so that should > > > alleviate some of these pain points somewhat. I'm not sure it is > > > clever enough to allow through writefulls for an ec base tier though > > > (but it would be a good > > idea!) - > > > > Excellent news, I shall look forward to testing in the future. I did > > mention the proxy write for write fulls to someone who was working on > > the proxy write code, but I'm not sure if it ever got followed up. > > I think someone here is me. In the current code, for an ec base tier, writefull > can be proxied to the base. Excellent news. Is this intelligent enough to determine when say a normal write IO from a RBD is equal to the underlying object size and then turn this normal write effectively into a write full? > > > > > > Sam > > > > > > On Tue, Aug 18, 2015 at 12:48 PM, Nick Fisk <nick@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > > >> -----Original Message----- > > > >> From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On > > > >> Behalf Of Mark Nelson > > > >> Sent: 18 August 2015 18:51 > > > >> To: Nick Fisk <nick@xxxxxxxxxx>; 'Jan Schermer' <jan@xxxxxxxxxxx> > > > >> Cc: ceph-users@xxxxxxxxxxxxxx > > > >> Subject: Re: any recommendation of using EnhanceIO? > > > >> > > > >> > > > >> > > > >> On 08/18/2015 11:52 AM, Nick Fisk wrote: > > > >> > <snip> > > > >> >>>> > > > >> >>>> Here's kind of how I see the field right now: > > > >> >>>> > > > >> >>>> 1) Cache at the client level. Likely fastest but obvious > > > >> >>>> issues like > > > > above. > > > >> >>>> RAID1 might be an option at increased cost. Lack of > > > >> >>>> barriers in some implementations scary. > > > >> >>> > > > >> >>> Agreed. > > > >> >>> > > > >> >>>> > > > >> >>>> 2) Cache below the OSD. Not much recent data on this. Not > > > >> >>>> likely as fast as client side cache, but likely cheaper > > > >> >>>> (fewer OSD nodes than client > > > >> >> nodes?). > > > >> >>>> Lack of barriers in some implementations scary. > > > >> >>> > > > >> >>> This also has the benefit of caching the leveldb on the OSD, > > > >> >>> so get a big > > > >> >> performance gain from there too for small sequential writes. I > > > >> >> looked at using Flashcache for this too but decided it was > > > >> >> adding to much complexity and risk. > > > >> >>> > > > >> >>> I thought I read somewhere that RocksDB allows you to move > > > >> >>> its WAL to > > > >> >> SSD, is there anything in the pipeline for something like > > > >> >> moving the filestore to use RocksDB? > > > >> >> > > > >> >> I believe you can already do this, though I haven't tested it. > > > >> >> You can certainly move the monitors to rocksdb (tested) and > > > >> >> newstore uses > > > >> rocksdb as well. > > > >> >> > > > >> > > > > >> > Interesting, I might have a look into this. > > > >> > > > > >> >>> > > > >> >>>> > > > >> >>>> 3) Ceph Cache Tiering. Network overhead and write > > > >> >>>> amplification on promotion makes this primarily useful when > > > >> >>>> workloads fit mostly into the cache tier. Overall safe > > > >> >>>> design but care must be taken to not over- > > > >> >> promote. > > > >> >>>> > > > >> >>>> 4) separate SSD pool. Manual and not particularly flexible, > > > >> >>>> but perhaps > > > >> >> best > > > >> >>>> for applications that need consistently high performance. > > > >> >>> > > > >> >>> I think it depends on the definition of performance. > > > >> >>> Currently even very > > > >> >> fast CPU's and SSD's in their own pool will still struggle to > > > >> >> get less than 1ms of write latency. If your performance > > > >> >> requirements are for large queue depths then you will probably > > > >> >> be alright. If you require something that mirrors the > > > >> >> performance of traditional write back cache, then even pure > > > >> >> SSD Pools > > can start to struggle. > > > >> >> > > > >> >> Agreed. This is definitely the crux of the problem. The > > > >> >> example below is a great start! It'd would be fantastic if we > > > >> >> could get more feedback from the list on the relative > > > >> >> importance of low latency operations vs high IOPS through > > > >> >> concurrency. We have general suspicions but not a ton of > > > >> >> actual data regarding what folks are seeing in practice and under > what scenarios. > > > >> >> > > > >> > > > > >> > If you have any specific questions that you think I might be > > > >> > able to > > > > answer, > > > >> please let me know. The only other main app that I can really > > > >> think of > > > > where > > > >> these sort of write latency is critical is SQL, particularly the > > > > transaction logs. > > > >> > > > >> Probably the big question is what are the pain points? The most > > > >> common answer we get when asking folks what applications they run > > > >> on top of Ceph is "everything!". This is wonderful, but not > > > >> helpful when trying to > > > > figure out > > > >> what performance issues matter most! :) > > > > > > > > Sort of like someone telling you their pc is broken and when asked > > > > for details getting "It's not working" in return. > > > > > > > > In general I think a lot of it comes down to people not > > > > appreciating the differences between Ceph and say a Raid array. > > > > For most things like larger block IO performance tends to scale > > > > with cluster size and the cost effectiveness of Ceph makes this a > > > > no brainer not to just add a handful of extra OSD's. > > > > > > > > I will try and be more precise. Here is my list of pain points / > > > > wishes that I have come across in the last 12 months of running Ceph. > > > > > > > > 1. Improve small IO write latency > > > > As discussed in depth in this thread. If it's possible just to > > > > make Ceph a lot faster then great, but I fear even a doubling in > > > > performance will still fall short compared to if you are caching > > > > writes at the client. Most things in Ceph tend to improve with > > > > scale, but write latency is the same with 2 OSD's as it is with > > > > 2000. I would urge some sort of investigation into the possibility > > > > of some sort of persistent librbd caching. This will probably help > > > > across a large number of scenarios, as in the end, most things are > > > > effected by latency > > and > > > I think will provide across the board improvements. > > > > > > > > 2. Cache Tiering > > > > I know a lot of work is going into this currently, but I will > > > > cover my experience. > > > > 2A)Deletion of large RBD's takes forever. It seems to have to > > > > promote all objects, even non-existent ones to the cache tier > > > > before it can > > delete > > > them. > > > > Operationally this is really poor as it has a negative effect on > > > > the cache tier contents as well. > > > > 2B) Erasure Coding requires all writes to be promoted 1st. I think > > > > it should be pretty easy to allow proxy writes for erasure coded > > > > pools if the IO size = Object Size. A lot of backup applications > > > > can be configured to write out in static sized blocks and would be > > > > an ideal candidate for this sort of enhancement. > > > > 2C) General Performance, hopefully this will be fixed by upcoming > > changes. > > > > 2D) Don't count consecutive sequential reads to the same object as > > > > a trigger for promotion. I currently have problems where reading > > > > sequentially through a large RBD, causes it to be completely > > > > promoted because the read IO size is smaller than the underlying > > > > object > > size. > > > > > > > > 3. Kernel RBD Client > > > > Either implement striping or see if it's possible to configure > > > > readahead > > > > +max_sectors_kb size to be larger than the object size. I started > > > > +a thread > > > > about this a few days ago if you are interested in more details. > > > > > > > > 4. Disk based OSD with SSD Journal performance As I touched on > > > > above earlier, I would expect a disk based OSD with SSD journal to > > > > have similar performance to a pure SSD OSD when dealing with > > > > sequential small IO's. Currently the levelDB sync and potentially > > > > other things slow this down. > > > > > > > > 5. iSCSI > > > > I know Mike Christie is doing a lot of good work in getting LIO to > > > > work with Ceph, but currently it feels like a bit of a amateur > > > > affair getting it going. > > > > > > > > 6. Slow xattr problem > > > > I've a weird problem a couple of times, where RBD's with data that > > > > hasn't been written to for a while seem to start performing reads > > > > very slowly. With the help of Somnath in a thread here we managed > > > > to track it down to a xattr taking very long to be retrieved, but no idea > why. > > > > Overwriting the RBD with fresh data seemed to stop it happening. > > > > Hopefully Newstore might stop this happening in the future. > > > > > > > >> > > > >> IE, should we be focusing on IOPS? Latency? Finding a way to > > > >> avoid > > > > journal > > > >> overhead for large writes? Are there specific use cases where we > > > >> should specifically be focusing attention? general iscsi? S3? > > > >> databases directly on RBD? etc. There's tons of different areas > > > >> that we > > > > can > > > >> work on (general OSD threading improvements, different messenger > > > >> implementations, newstore, client side bottlenecks, etc) but all > > > >> of those things tackle different kinds of problems. > > > >> > > > >> Mark > > > >> _______________________________________________ > > > >> 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 > > > _______________________________________________ > > > 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 > _______________________________________________ > 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
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