FW: Flash or SSD for MultiBlock Reads
- From: "Mark W. Farnham" <mwf@xxxxxxxx>
- To: "'ORACLE-L'" <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 11 Sep 2014 11:09:05 -0400
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).
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
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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