Re: Flash or SSD for MultiBlock Reads

Like I always say, "Everything is a CPU problem."



________________________________
 From: "Chitale, Hemant K" <Hemant-K.Chitale_at_sc.com>
To: mwf_at_rsiz.com 
Cc: ORACLE-L <oracle-l_at_freelists.org> 
Sent: Wednesday, September 17, 2014 6:50 PM
Subject: RE: Flash or SSD for MultiBlock Reads
 


>Well configured SSD can saturate the plumbing or CPU
I agree.  I see that happening when we move from disks to the new technology (I don’t know what Flash/SSD/Vendor technology it really is, details aren’t visible to me).  We’ve had to add more CPU cores to handle the I/O being returned by PQ slaves.   (Earlier, disk being the bottleneck, CPUs were idle, now they are “in demand”).
 
Hemant K Chitale
 
 
From:oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce@freelists.org] On Behalf Of Mark W. Farnham
Sent: Wednesday, September 17, 2014 3:55 PM
To: 'Kevin Closson'; frits.hoogland_at_gmail.com
Cc: 'ORACLE-L'
Subject: RE: Flash or SSD for MultiBlock Reads
 
I *think* Kevin and my analyses are ultimately exactly the same. Well configured SSD can saturate the plumbing or CPU with either single block or multi-block read, direct or buffered, but you might need parallelism of demand to fully load the plumbing if you have good plumbing.
 
Also in the context of the OP about the difference: There is more advantage for single block because there is more seek to eliminate, which is latency eliminated even at 100% bandwidth. That does not make single block SSD reads faster than multiple block SSD reads. It makes single block SSD a greater proportional improvement over single blocks from spinning rust than multi-block SSD is over multiple blocks from spinning rust.
 
mwf
 
From:Kevin Closson [mailto:ora_kclosson_at_yahoo.com] 
Sent: Tuesday, September 16, 2014 7:02 PM
To: frits.hoogland_at_gmail.com; dmarc-noreply@freelists.org
Cc: mwf_at_rsiz.com; ORACLE-L
Subject: Re: Flash or SSD for MultiBlock Reads
 
I'm only responding to the OP which is MBRC in the context of Non-Mechanical storage. There are so many host ramifications that have to do with MBRC, yes, but my assertion is the size of MBRC *from modern all flash perspective* doesn't matter nearly as much as it does in mechanical storage. And as my case in point suggests I get saturate storage plumbing with low MBRC or high MBRC and that actually goes for conventional and direct. Now, having said all that I'll loop back to my mention of short tables scans. I really don't think it is optimal at the app level to wait for, say, 2 PIO on a really short table scan when perhaps the whole small table could have been read in a single I/O.
 
In general there are so many things we know about how Oracle evolved to optimize out the pain suffered by physical high latency I/O. It's good to remember old facts, but things change too...
 
So what's the reader to do? Well, I'll put it this way. Test *always* with the defaults Oracle sets. If you aren't perfectly happy with that then consider taking action.
 
 

________________________________

From:Frits Hoogland <frits.hoogland_at_gmail.com>
To: dmarc-noreply_at_freelists.org 
Cc: "mwf_at_rsiz.com" <mwf@rsiz.com>; ORACLE-L <oracle-l@freelists.org> 
Sent: Tuesday, September 16, 2014 2:16 PM
Subject: Re: Flash or SSD for MultiBlock Reads
 
There is a big, inherent, difference between the multiblock reads done buffered (scattered read) or to PGA (direct path read), because direct path reads are automatically and adaptively being done in parallel outside of parallel query.
 
But I think you are talking about direct path reads I guess Kevin? 

Frits Hoogland

http://fritshoogland.wordpress.com
frits.hoogland_at_gmail.com
Office : +31 20 5939953
Mobile: +31 6 14180860
 
 
On 16 Sep 2014, at 23:06, Kevin Closson (Redacted sender "ora_kclosson_at_yahoo.com" for DMARC) <dmarc-noreply@freelists.org> wrote:
 
multiblock reads make it easy to suffer storage networking bottlenecks but since these devices are seek free it so happens to be the case that usually one can obtain the same throughput with, say, 32K reads as with 1MB. Be aware however, it depends on how the firmware of the device works. In other words, mileage varies. 
 
I can speak of devices I'm familiar with. For example, EMC XtremIO with PQO MBRC=8/db_block_size=8K will saturate the 4x8GFC (~95% of max theoretical) plumbing (per X-Brick) just as easily as MBRC=64 or 128.
 
 

________________________________

From: Mark W. Farnham <mwf_at_rsiz.com>
To: 'ORACLE-L' <oracle-l_at_freelists.org> 
Sent: Thursday, September 11, 2014 8:09 AM
Subject: FW: Flash or SSD for MultiBlock Reads
 
forgot the list.
 
From: Mark W. Farnham [mailto:mwf_at_rsiz.com] 
Sent: Thursday, September 11, 2014 12:44 AM
To: 'Hemant-K.Chitale_at_sc.com'
Subject: RE: Flash or SSD for MultiBlock Reads
 
For direct single block reads you have a chance to have a seek plus read for each single block read. For direct multiblock reads when the size is reasonably matched to the disk you probably get only 1 or 2 seeks (2 when you hit a boundary in the middle of the read).
 
Since seeking is the biggest advantage of SSD over spinning rust, that means the advantage of SSD is likely maximized for single block reads.
 
For writes it is complicated by the page write size, but we’re only talking about reads here, right? On file systems you get some reduction in seeks because there is probably prospective next block buffering into the file system cache (but you pay for that overall by the move from cache to cache, which is why folks like direct, although you probably don’t dodge all levels of cache on a modern disk farm.)
 
But marginally for most workloads the advantage of SSD is diminished a bit for multiblock reads because there are fewer seeks per volume of data moved. For parallel you’ve got to tell us how fat your communications pipes are. You can probably multi-read from SSD faster than your channels can absorb and cpus can process, so there is a ceiling to the advantage of ssd in the parallel model.
 
Kevin Closson has been publishing some “holy cow that is fast” measurements of a certain combination of hardware kit. With the right configuration of the stuff he is using, it appears data loading is gated on cpu speed. (We always have some pacing resource. When it is memory movement or cpu, that is pretty much as fast as it can get).
 
mwf
 
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce@freelists.org] On Behalf Of Chitale, Hemant K
Sent: Wednesday, September 10, 2014 11:24 PM
To: ORACLE-L
Subject: Flash or SSD for MultiBlock Reads
 
Are there any “thumb-rules” or “guidances” about the performance improvement of Flash / SSD over rotating disks when serving multiblock reads  and/or parallel query full-table-scans ?
That is, is the performance gain of Flash / SSD over rotating disks the same or better or worse when doing multiblock reads versus when doing single block reads.
Hemant K Chitale

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