Re: question for the new ceph-osd key/value backend

[Mark Nelson <mark.nelson@inktank.com>]

On 12/11/2013 12:59 AM, Duan, Jiangang wrote:
> Thanks. I in general think to find one implementation suitable for all usage models (small vs. big, cold vs. hot) is very difficult. So I like the idea of "a backend that lets you plug in a next-gen backend beneath it - " -
> K/V may be a good way to handle many small objects than XFS - however I am not sure if levelDB is the right choice (consider it is good for write than read) and also not sure K/V this will benefit RBD workload or not (consider all 4MB object size).
> Will think more about this and talk with you again.

I have been very interested in this topic recently and have been doing 
some benchmarking with basho's leveldb, hyperdex, and stock leveldb 
implementations.  Each has certain advantages (usually related to 
whatever they advertise, ie crc32 for basho, etc), but all seem to have 
poor sync read/write performance, both sequential and random.  I want to 
take a look at rocksdb to see how it compares as well.

This whole area is very interesting as there are obvious tradeoffs 
regarding how we do things now vs what we potentially could do down the 
road.  Being able to eliminate POSIX entirely behind the scenes would 
obviously be nice for a lot of reasons.

Mark

>
> -jiangang
>
> -----Original Message-----
> From: Sage Weil [mailto:sage@inktank.com]
> Sent: Wednesday, December 11, 2013 2:09 PM
> To: Duan, Jiangang
> Cc: ceph-devel@vger.kernel.org
> Subject: Re: question for the new ceph-osd key/value backend
>
> Hi Jiangang,
>
> On Wed, 11 Dec 2013, Duan, Jiangang wrote:
>> Sage,
>>
>> I have some questions regarding to the key/value backend work.
>>
>> What is the motivation to work on this? (or what is the problem we
>> want to solve?)
>> 1) to use the new interface thus we can bypass all the OS layer thus get a short latency?
>
> That is one part.  The current strategy of layering on top of a file system and using a write-ahead journal makes sense given the existing linux fs building blocks, but is far from an optimal solution for many workloads.  A k/v interface based on something leveldb probably performs much better for many small-object use-cases.  Also, a k/v backend can take advatange of emerging non-block storage interfaces like NVMKV, Kinetic, new libraries like rocksdb, etc.
>
>> 2) or to leverage some new primitive e.g. the atomic write thus to simplify the code writing?
>
> That too.  Basically, we are currently doing a lot of work to get what we need out of posix, and are paying the price.
>
>> There are several different possibilities to use future NVM technology
>> - NVM.FILE, NVM.BLOCK, PM.XXX
>> http://snia.org/sites/default/files/NVMProgrammingModel_v1r10DRAFT.pdf
>> Even for openNVM thing - there are other usage model than k/v.
>>
>> Do you have any typical usage model for this?
>
> I wasn't familiar with these; thanks for the reference!  Of these, NVM.FILE seems the most interesting (it maps most closely to an object).
> I am predisposed to skepticism when it comes to these sorts of standards/API docs that precede an actual implementation, but it is encourgaging to see some effort here towards a common interface.
>
> In the end, we want to support generic Ceph workloads.  These range from rbd block and file type workloads (objects are stripes of files, with random bytes rewritten) to omap type workloads (like rgw bucket indices that are purely key/value).
>
> I think the first wins would be:
>
> 1- a backend that more efficiently handles rgw bucket index workloads
> 2- a backend that is more efficient for rgw in general (i.e., immutable
> objects)
> 3- a backend that can handle more general purpose workloads (like rbd and
> cephfs)
>
> and separately,
>
> 4- a backend that lets you plug in a next-gen backend beneath it, like NVMKV and speedy flash.
>
> sage
>
>
>
>>
>> -jiangang
>>
>> ===================
>>
>> From: Sage Weil <sage <at> inktank.com>
>> Subject: new ceph-osd key/value backend
>> Newsgroups: gmane.comp.file-systems.ceph.devel
>> Date: 2013-11-09 10:09:52 GMT (4 weeks, 3 days, 16 hours and 39
>> minutes ago) I've written up a blueprint with a rough sketch of how to
>> take advantage of alternative storage interfaces.  I am very happy to
>> see that several f them have emerged over the past year or two:
>>
>>   - fusionio's KVMKV is a key/value interface for their flash products
>>   - seagate's kinetic is a key/value interface for their new
>> ethernet-based drive
>>
>> Also, leveldb is pretty great for many workloads when run on a
>> tranditional disk/fs.
>>
>> The good news is a lot of the existing work that went into support
>> omap looks to be reusable here.  Some new functionality and
>> refactoring is needed, though, particularly when it comes to storing
>> object data (the file-like bag of bytes portion) as key/value pairs.
>>
>> The blueprint is here:
>>
>>
>> http://wiki.ceph.com/01Planning/02Blueprints/Firefly/osd%3A_new_key%2F
>> %2Fvalue_backend
>>
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