Re: Flash or SSD for MultiBlock Reads

  • From: "Kevin Closson" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "ora_kclosson@xxxxxxxxx" for DMARC)
  • To: "mwf@xxxxxxxx" <mwf@xxxxxxxx>, 'ORACLE-L' <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 16 Sep 2014 14:06:17 -0700

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@xxxxxxxx>
To: 'ORACLE-L' <oracle-l@xxxxxxxxxxxxx> 
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@xxxxxxxx] 
Sent: Thursday, September 11, 2014 12:44 AM
To: 'Hemant-K.Chitale@xxxxxx'
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).
From:oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On 
Behalf Of Chitale, Hemant K
Sent: Wednesday, September 10, 2014 11:24 PM
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

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