Re: Flash or SSD for MultiBlock Reads

From: Kevin Closson <ora_kclosson_at_yahoo.com>
Date: Tue, 16 Sep 2014 14:06:17 -0700
Message-ID: <1410901577.92917.YahooMailNeo_at_web121206.mail.ne1.yahoo.com>


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
 


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_at_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 / SSDover rotating disksthe same or better or worse when doing multiblock readsversus when doing single block reads.
Hemant K Chitale

This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https://www.sc.com/en/incorporation-details.html.
--
http://www.freelists.org/webpage/oracle-l
Received on Tue Sep 16 2014 - 23:06:17 CEST

Original text of this message