I think you are right that if you replicate the row MVCC semantics in the cache, then you can get a consistent view. I was referring to a more client side approach.
I guess the take aways are: - forget about scans, and shoot for point gets, which I agree - per-kv cache overhead might be huge, but still worth trying it out. - can also be architected on top of coprocessors - might be complex to implement, but still some use cases would benefit a lot. - row cache / family cache / kv cache Thanks for all the input! Enis On Wed, Apr 4, 2012 at 3:28 PM, Matt Corgan <[email protected]> wrote: > A client-side memcached setup can stay pretty consistent if you send all of > your puts and deletes through it before sending them to hbase, but yeah, I > guess you lose strict consistency under heavy read/write from multiple > simultaneous clients. But, like Andy is saying, if you route the requests > through the regionserver and it talks to memcached/hazelcast, couldn't that > be fully consistent? > > > On Wed, Apr 4, 2012 at 3:09 PM, Andrew Purtell <[email protected]> > wrote: > > > I thought about trying this out once with a coprocessor, hooking the > Gets, > > with an embedded Hazelcast. That would just be a proof of concept. The > idea > > is to scale the KV cache independent of regionserver limits (maybe we're > > only giving 1 GB per RS to the value cache and a 10 GB region is hot) and > > the next step could be modifying the client to spread read load over > > replicas (HBASE-2357). This doesn't consider scans either. > > > > > > Best regards, > > > > - Andy > > > > > > Problems worthy of attack prove their worth by hitting back. - Piet Hein > > (via Tom White) > > > > > > > > >________________________________ > > > From: Matt Corgan <[email protected]> > > >To: [email protected] > > >Sent: Wednesday, April 4, 2012 2:46 PM > > >Subject: Re: keyvalue cache > > > > > >It could act like a HashSet of KeyValues keyed on the > > >rowKey+family+qualifier but not including the timestamp. As writes come > > in > > >it would evict or overwrite previous versions (read-through vs > > >write-through). It would only service point queries where the > > >row+fam+qualifier are specified, returning the latest version. Wouldn't > > be > > >able to do a typical rowKey-only Get (scan behind the scenes) because it > > >wouldn't know if it contained all the cells in the row, but if you could > > >specify all your row's qualifiers up-front it could work. > > > > > > > > >On Wed, Apr 4, 2012 at 2:30 PM, Vladimir Rodionov > > ><[email protected]>wrote: > > > > > >> 1. 2KB can be too large for some applications. For example, some of > our > > >> k-v sizes < 100 bytes combined. > > >> 2. These tables (from 1.) do not benefit from block cache at all (we > did > > >> not try 100 B block size yet :) > > >> 3. And Matt is absolutely right: small block size is expensive > > >> > > >> How about doing point queries on K-V cache and bypass K-V cache on > all > > >> Scans (when someone really need this)? > > >> Implement K-V cache as a coprocessor application? > > >> > > >> Invalidation of K-V entry is not necessary if all upserts operations > go > > >> through K-V cache firstly if it sits in front of MemStore. > > >> There will be no "stale or invalid" data situation in this case. > > Correct? > > >> No need for data to be sorted and no need for data to be merged > > >> into a scan (we do not use K-V cache for Scans) > > >> > > >> > > >> Best regards, > > >> Vladimir Rodionov > > >> Principal Platform Engineer > > >> Carrier IQ, www.carrieriq.com > > >> e-mail: [email protected] > > >> > > >> ________________________________________ > > >> From: Matt Corgan [[email protected]] > > >> Sent: Wednesday, April 04, 2012 11:40 AM > > >> To: [email protected] > > >> Subject: Re: keyvalue cache > > >> > > >> I guess the benefit of the KV cache is that you are not holding entire > > 64K > > >> blocks in memory when you only care about 200 bytes of them. Would an > > >> alternative be to set a small block size (2KB or less)? > > >> > > >> The problems with small block sizes would be expensive block cache > > >> management overhead and inefficient scanning IO due to lack of > > read-ahead. > > >> Maybe improving the cache management and read-ahead would be more > > general > > >> improvements that don't add as much complexity? > > >> > > >> I'm having a hard time envisioning how you would do invalidations on > > the KV > > >> cache and how you would merge its entries into a scan, etc. Would it > > >> basically be a memstore in front of the memstore where KVs get > > individually > > >> invalidated instead of bulk-flushed? Would it be sorted or hashed? > > >> > > >> Matt > > >> > > >> On Wed, Apr 4, 2012 at 10:35 AM, Enis Söztutar <[email protected]> > wrote: > > >> > > >> > As you said, caching the entire row does not make much sense, given > > that > > >> > the families are by contract the access boundaries. But caching > column > > >> > families might be a good trade of for dealing with the per-item > > overhead. > > >> > > > >> > Also agreed on cache being configurable at the table or better cf > > level. > > >> I > > >> > think we can do something like enable_block_cache = true, > > >> > enable_kv_cache=false, per column family. > > >> > > > >> > Enis > > >> > > > >> > On Tue, Apr 3, 2012 at 11:03 PM, Vladimir Rodionov > > >> > <[email protected]>wrote: > > >> > > > >> > > Usually make sense for tables with random mostly access (point > > queries) > > >> > > For short-long scans block cache is preferable. > > >> > > Cassandra has it (Row cache) but as since they cache the whole row > > >> (which > > >> > > can be very large) in many cases > > >> > > it has sub par performance. Make sense to make caching > configurable: > > >> > table > > >> > > can use key-value cache and do not use block cache > > >> > > and vice verse. > > >> > > > > >> > > Best regards, > > >> > > Vladimir Rodionov > > >> > > Principal Platform Engineer > > >> > > Carrier IQ, www.carrieriq.com > > >> > > e-mail: [email protected] > > >> > > > > >> > > ________________________________________ > > >> > > From: Enis Söztutar [[email protected]] > > >> > > Sent: Tuesday, April 03, 2012 3:34 PM > > >> > > To: [email protected] > > >> > > Subject: keyvalue cache > > >> > > > > >> > > Hi, > > >> > > > > >> > > Before opening the issue, I though I should ask around first. What > > do > > >> you > > >> > > think about a keyvalue cache sitting on top of the block cache? It > > is > > >> > > mentioned in the big table paper, and it seems that zipfian kv > > access > > >> > > patterns might benefit from something like this a lot. I could not > > find > > >> > > anybody who proposed that before. > > >> > > > > >> > > What do you guys think? Should we pursue a kv query-cache. My gut > > >> feeling > > >> > > says that especially for some workloads we might gain significant > > >> > > performance improvements, but we cannot verify it, until we > > implement > > >> and > > >> > > profile it, right? > > >> > > > > >> > > Thanks, > > >> > > Enis > > >> > > > > >> > > Confidentiality Notice: The information contained in this > message, > > >> > > including any attachments hereto, may be confidential and is > > intended > > >> to > > >> > be > > >> > > read only by the individual or entity to whom this message is > > >> addressed. > > >> > If > > >> > > the reader of this message is not the intended recipient or an > > agent or > > >> > > designee of the intended recipient, please note that any review, > > use, > > >> > > disclosure or distribution of this message or its attachments, in > > any > > >> > form, > > >> > > is strictly prohibited. If you have received this message in > error, > > >> > please > > >> > > immediately notify the sender and/or > [email protected] > > >> > > delete or destroy any copy of this message and its attachments. > > >> > > > > >> > > > >> > > >> Confidentiality Notice: The information contained in this message, > > >> including any attachments hereto, may be confidential and is intended > > to be > > >> read only by the individual or entity to whom this message is > > addressed. If > > >> the reader of this message is not the intended recipient or an agent > or > > >> designee of the intended recipient, please note that any review, use, > > >> disclosure or distribution of this message or its attachments, in any > > form, > > >> is strictly prohibited. If you have received this message in error, > > please > > >> immediately notify the sender and/or [email protected] and > > >> delete or destroy any copy of this message and its attachments. > > >> > > > > > > > > > > > >
